Yeah, I heard Vista sucks and that's what my system uses. Plus, I've read negative reviews about Vista and had no idea what was being talked about. What is the difference between Vista and XP???

Vista is flashy and more secure than XP.
Read: More secure. It's up-to-date. XP is 8 years old. 8 years old.
I don't get where the negative reviews about Vista come from. I see nothing wrong with it.

Vista is more flashy and an immense resource hog.
If you have a powerful system by all means go with Vista. If not, stick to XP.
Saying that XP is not up to date is just bullcrap. XP has been constantly kept up to date through service packs and patches in the 8 years since it's original release.

The differences are really too numerous to list, Vista is a completely different operating system from XP. Microsoft released a white paper comparison of the two systems that you can download here, if you want a really boring read.

It is available on Windows XP (The Games for Windows Live)

Ah right I had no idea. I guess its just for any game which has achievement points.

AFAIK, GFWLive is exactly the same on XP and Vista. The only real difference is in the games themselves, as the DX10 games have to revert to DX9 on XP.

Are you forced to use Games for Windows Live at all?
I'd hate it to be something extra to clog up my system. I try and run quite a tight ship!
I have this fear that when I upgrade, I'll be forced to have GfWL on, which would annoy me as much as Steam used to before I exorcised it from my system.

Well, if you buy a Steam game, you're forced to use Steam. I don't have any GfWL games, so I'm not 100% positive, but I think that if you buy a GfWL game, you're forced to use WL. And anything else the game requires. I've heard horror stories about GTA IV, which supposedly requires all sorts of other crap as well.

GFWL seems to have a very small footprint, seems to make no difference to my laptop and its a slow old beast. Then again I think the same about steam...
GTA4 apparently needed the rockstar social gaming thing along with GFWL which did seem excessive

It depends on the game. Some it's optional for certain features (online play, gamersscore, etc), for some it is required for online activation, install limits, etc.

Single-player-only games (such as Fallout 3) only use Live for downloadable content and achievements; if you don't care about either of those you can play without it and ignore it completely. Unfortunately this does not seem to apply to games that use it for multiplayer (such as Grand Theft Auto IV) even if you only care about the single-player component, although of course you could use a crack or whatever to get around that. Games for Windows Live is not bundled with Windows (not even Windows 7), so if you never buy a Live game you will never have to deal with it.
Any and all Games for Windows titles that do not bear the Live logo have no Live functionality whatsoever; for these titles, all the branding does is guarantee they follow the design guidelines (optional single-click installation, controller support, Games Explorer integration, widescreen resolutions, 64-bit support, etc.), although these are things that many non-branded games do anyway.
DirectX 10 was a relatively minor release to expand DirectX with new features originally introduced with the Xbox 360's DirectX-derived API. Additionally, since all games continue to support DX9 as well (with very few exceptions, like Stormrise and the upcoming Alan Wake) developers' efforts are divided between the two, meaning DX10 didn't get all the attention it could have.
DirectX 11, on the other hand, is a fairly major update, adding next-generation features such as tessellation (which allows the detail level of each model to be scaled mathematically rather than having to swap between multiple pre-designed versions of the each model) and compute shaders (which allows non-graphics tasks--such as physics--to be handled on the GPU using manufacturer-independent code).
Tesselation means the developers only need to have one version of each model (with the engine and GPU scaling the detail in real time during play) while also theoretically allowing games to scale to a wider range of hardware than ever before. DX11 also incorporates feature set scaling for DX10 and 9 cards, which means developers can shift to working entirely in DX11 without dropping support for older cards in the process. Because of this it is very likely that many developers will skip DX10 altogether, especially since the next Xbox is very likely to run on a modified version of DX11.

Thanks folks,
I do have Fallout 3, and it makes very little mention of the Live component, and I haven't had to use it at all (I won't be buying any of the DLC).
I was avoiding GTA4 anyway so thats not a big deal. I'm a single-player gamer, so all this sort of guff gets in my way. Hopefully I won't run into any games that will require it, the only new games I'm likely to get this year are Diablo 3 and Dragon Age, and I would be very suprised if either of them required it.