The solution I tlaked about in my post may have worked : it's currently looking for updates, wchich is a huge improvement. We'll see :)
Edit : Holy Guacamole it worked !!!
Here's the method I followed (so for Windows updates fails with error 80070bc9) :
1) Open cmd and type :
fsutil resource setautoreset true c:\
2) Reboot in safe mod
3) open cmd and type this commands :
C:\WINDOWS\system32>cd %SystemRoot%\System32\SMI\Store\Machine
C:\WINDOWS\System32\SMI\Store\Machine>attrib -s -h *
C:\WINDOWS\System32\SMI\Store\Machine>del *.blf
C:\WINDOWS\System32\SMI\Store\Machine>del *.regtrans-ms
C:\WINDOWS\system32\SMI\Store\Machine>cd %SystemRoot%\System32\config\TxR
C:\WINDOWS\System32\config\TxR>attrib -s -h *
C:\WINDOWS\System32\config\TxR>del /q *
4) Reboot, and it should work

Just did and answered you :) (but maybe you haven't received my answer depending on your privacy settings since we aren't "friends", so I sent you a friends request too)

Yes. But I don't know what softwares you have in mind, maybe you include games here? I have around 12 GB of software and is all on the windows partition. Even if all was portable, still not worth a dedicated partition.

I go for portable versions whenever I have the option. Opera, Pale Moon, Skype, VLC, Thunderbird, all Piriform's programs and most of the freeware software I use is portable (I should do the same for Firefox but I'm lazy). And don't put them in C:\Program Files (x86)\ or C:\Program Files\.

It depends a lot on how someone is managing their files. Having multiple partitions can equal performance, flexibility and less time doing maintenance.

For example, if I copy a big unfragmented file from a partition at the beginning of my drive, it has around 120 MB/s transfer speed. Same scenario, but from a partition at the end of the drive is only around 65 MB/s. So it's definitely better to have Windows and games on the first partitions

Disk read/write heads will travel less back and forth on a smaller partition, thus having better access times.

Different cluster sizes for partitions. On a windows partition and one with lots of smaller files, like games (looking at Shadowrun Dragonfall with over 23000 files O_o) it's best to have the default 4 KB cluster size, but on one with big files, like movies, archives, ISOs, the performance will be increased with bigger cluster sizes (e.g., 32 KB).

Having to do an error-checking for bad sectors on a single partition drive can take a huge amount of time. I once did that on a newly bought 2 TB external drive and it took around 12h - learned my lesson.
Smaller Windows partitions (120-200 GB) can easily be cloned and restored when needed.

Less fragmentation with multiple partitions; Windows likes to scatter its files all over the place. I have all my music on a single partition and it needs no defragmenting because I rarely do writing on it.
Btw, Defraggler has an option to move specific files at the end of drive/partition, so you can do that for rarely accessed files (like those from C:\Windows\Installer\, for example) - don't know why the opposite isn't available too.

I kinda do that too, but that "data partition" is on a completely different hard drive. Either on the secondary internal hard drive, or even (or additionally) an external USB hard drive.

So my first drive is still in one partition, and there I install my OS and all (most) applications. Then the separate drive, D:, is where I keep music, photos, videos, installers etc.

Yeah, I know it's a bit a "messy" way to do, but I install lot of games & software into the windows partition :P

This is very interesting, I take note.
Having to do an error-checking for bad sectors on a single partition drive can take a huge amount of time. I once did that on a newly bought 2 TB external drive and it took around 12h - learned my lesson.
I didn't thought about cloning, that's right ! I'll really keep this in mind and think about it for a next time.

Okay, I received my disk ! (yay !) and plugged it successfully (yay !!!) And got an image thanks to ariaspi who send me the official link I needed (thanks pal !).
Question : Unfortunately, windows still wants me to do the install with "UEFI" instead of usual Bios. I mean it's permitting to install windows if I want, but with that EFI thing which brought so much trouble when I tried to solve my situation earlier (preventing me to use live DVD of ubuntu, kaspersky recue key for example) : is there a way to avoid that ? I looked through options of my bios and can't find the option to deactivate "secure boot" (I can only create a password in a "security" menu). Maybe I would need to update my bios ?
Or maybe I should simply install windows without worrying about it, and accept UEFI ? (it only means I won't be able to boot on live dvd if I need to...)
This is the safest way, to have a second drive :)
But with 2To of space, I have enough space to store a lot of things, so 2 partitions is the way for me :) (I prefer, even only for defragmentation :P)

To me it normally means doing more maintenance, like moving big amount of files from one partition to another in order to make room to one partition. If those two partitions were already joined, then such move operations wouldn't be necessary.

Also when installing the OS first time, I need to figure out what should be an optimal size for the OS partition. Do I try to make it as small as possible so that I don't waste hard drive space on something which I could use for applications or data files? Will it be enough also years from now when the OS partition gets cluttered of temp files, restore points and old driver packages (I'm always amazed how much space the old NVidia Geforce driver packages take, since the Nvidia utility keeps them around), or when Microsoft releases some SP or "Anniversary update" which needs a lot of free room to install?

If I just keep it all in one partition, I don't have to think about any of those, nor do I feel I am being wasteful because some partition has unused free space I would like to use for some other purpose (be it applications or data files).

Also, some file operations like "move" are far much faster when you operate within the same (bigger) partition. So if I move 200GB of files from one folder to another, it happens almost instantly if those two folders are within the same parition, but it takes ages if they are on two different partitions.

Also what you said about fragmentation, will it not just increase fragmentation if you have e.g. 5 + 5 + 5 GB of free space on three separate partitions, instead of having 15 GB of free space on one bigger partition? I recall some defragmentation utilities even refuse to work if you have too little free space on the partition.

I guess in the end I became so much against the idea of dividing one hard drive into several partitions after those "too small OS partition" problems on couple of friends' PCs. I couldn't figure out why someone had felt that one hard drive (which wasn't that big to begin with) should be divided into three partitions, which caused the OS partition to be so small that after certain point it couldn't download and install Windows updates anymore. Apparently Windows had grown over the years so much that the OS partition which seemed big enough years ago, wasn't big enough anymore.

Yeah there are utilities to change partition sizes on the fly, but that is yet another form of extra maintenance...

But to each his own, of course. On Linux, I like to have the swap partition as a separate one, because... well, Linux folks seem to think it is a good idea to have the swap file on its very own partition.

Okay, I'm trying a bit to see the options of installation of windows... and I suddenly remember that I should format the drive in ntfs before installing windows (because I'm getting an error and I just understood it's certainly becaus of that :P). How to do that with the windows installation dvd ? Do I need dos commands who are in "repair comp'? Is it possible to do that easily with dos ?

Does it not recognise the new disk? First partition, then format.

Windows installation sees my hdd. It permits me to create partitions. Even "format". But when I want to click on "next" to validate and go to next step, it says (beware, french to english translation) :
"Windows can't beinstalled into this disk. The hardware of this computer can't take in charge the start on this disk. Check that the controller/supervisor [not sure at all of the traduction for this word] of this disk is activated in the Bios menu of the computer."
Hdd seems well recognized on the bios (but I may be mistaking). But it's maybe too again the UEFI boot who's messing things up.
For info, I'm following this tutorial :which say How to Install Windows 7 Using the "Unified Extensible Firmware Interface" (UEFI) and this tutorial : Clean Install Windows 7.
I'm stuck at the step 7 of UEFI tuto.

I agree, but the OP already said that he doesn't want to spend more money on a 2nd HDD.

I'm very ignorant about UEFI but.. I think I managed to install Win7 with it by disabling Secure Boot..

EDIT: wait a second I just remembered that Win7 couldn't boot from 4K sectors native drives..

Unfortunately, I can't find any option in my bios to disable it :S
It's really the new standard ?
Edit :
What does that mean ? "Too big" drives ?

Yeah, what do you mean? O_o 4 KB cluster size? That's the default size for NTFS.

Edit:
From what I understand, BIOS can't handle bigger than 2 TB drives, so UEFI resolves that.

Edit2:
Oh, I see now what you mean.

Edit3:
I think it's okay for Windows 7 with SP1.

Sorry, I'm going by memory.. I did this thing 2 years ago. :\

I think that you need UEFI if you want GPT.
This will allow you to use drives bigger than 2.2TB.
(MBR could do that only with Advanced Format drives aka 512E)

About the sectors, I have to look it up, but can you check somehow if your model is 4K native or 512E?

No problem, I perfectly understand ;) (when you don't practise this kind of things regularly.. it's hard to keep it fresh in memory)
Ok so I'll need that since I have an external drive of 4To. (If I understand correctly)
How the hell can I do that ? XD It was [url=https://www.rueducommerce.fr/produit/toshiba-p300-high-performance-hard-drive-2tb-bulk-27378234#moid:MO-78DA6M66697524]this drive[/url] :P

Okay, but I shouldn't have formatted the hdd BEFORE plugging it into my comp' ?
Edit : I got an answer : nope.

So why is windows saying it can't install on my disk damn it ? XD

Ops, I made some errors, check my last edit (and please modify the quote too).

About the last point: it could be that, I'm not sure.

I can check the native physical size of my current drives with the command:
fsutil fsinfo ntfsinfo c:

As expected, mine is 512.
You could check yours from the manufacturer specs (you'll need the model code) or by plugging the HDD in another PC.
Or maybe you could run such command with a live CD.

--

EDIT

I've found another possible cause:
did you check if your SATA mode is AHCI and not IDE?