Just for curiosity (and partly cause of boredom) I will ask you people what operating system are you using and why? Any problems with it? Does it have anything special that makes it really stand out from other operating systems? Speak, people!
I use x86 Vista Home Premium and I just love it, though it has some gripes, like some random compatibility problems with some applications, UAC, sidebar (I don't have any use for it) and stuff like that. The user interface is imo a lot easier to use than one in xp and I especially like the search bar in start (or orb)-menu. That small thing makes my life a lot easier. Other thing start-menu has is that all the applications are in one list instead of bloating the whole screen with your installed software.

Well, I am using Windows 7 x64. Mostly because I got it free. I really hate UAC so I turned it off. One thing I like a lot is the new taskbar. On the programs side everything I have installed is running just fine. Some even better like City of Heroes that had some vista issues now runs just fine.

Running Windows 7 RTM x64 on my ASUS G71G laptop.
It has been running flawlessly for almost a week now (since I installed it). Had few issues with the drivers, but they were dealt with by installing them in Vista compatibility mode.
And... I am a happy Windows 7 user. Microsoft have really outdone themselves with this OS.

Yeah, I also have UAC tuned up to the maximum protection level. I want to know exactly what the software is trying to do. Better to be safe than sorry. :)

Just regular XP Pro with a few tweaks provided by TweakUI. I've pre-ordered Win7 because of the pre-order price reductions. But I in no way predict it's going to become my primary OS for some time. Probably not until the first service pack if the RC is anything to go by.

Leopard (but Snow Leopard should be arriving in the mail any day now.)

Note that UAC in 7 has far fewer prompts if you leave it at its default setting (although you can turn it up to a Vista-style prompting level if you so wish).
I am running Windows 7 64-bit and I am very happy with it. It's by far my favourite version of Windows ever and I will definitely be buying it as soon as it becomes available. It is an improvement over Vista in every regard, and if you are coming from XP the changes are even more significant.

Windows XP for the win!
But I will definitely be upgrading to Windows 7 once it comes out.

Windows XP - thats what this aging PC came with, and I don't see anything that would persuade me to pay to upgrade to Vista. Our next PC will probably have Vista (maybe with a free upgrade to Win7) but thats will be a laptop to be primarily used by my daughter, this will remain my primary gaming machine.

Home:
* Windows 7 RC x64, no real customisations I can think of other than a few basic built-in options.
* Mac OS X Leopard, changed the dock back to 2D style (so even if it sits on the bottom, it's got the same style as if it was on the left or right side), and removed the menu bar transparency.
Work:
* Windows Vista Business x86, with SharpEnviro.

I'm going to be different.
I use Gentoo Linux on my desktop, Fedora 11 on my laptop, and Arch Linux on my Eee PC.
However, my desktop duals boots Vista too. I couldn't get XP to install. It wouldn't let me use my keyboard. :(

Win XP Home. Runs what I want to run and stays out of my way. I'll probably switch to Win 7 whenever I build my next computer, but that's at least a year off (probably more like 2 years).

I've got an XP Pro, an XP Home, an XP Media Centre Edition & Vista Ultimate x64. Oh and Windows Mobile 6.1
The XP Pro is on my download computer which is a clunky old piece of crap, 1ghz athlon with 256mb ram so thats stripped down to the minimum, all nonessential services turned off and everything shiny turned off. Its also a headless system so I need the maximum resources to run the remote desktop app
The XP Home came with my netbook and has only had the basic stuff I do to most of my PCs done, turning back to the classic start menu, changing the window background to light grey instead of white and making each folder open in its own window
The XPMCE has basically sat untouched since I finished installing the OS and codecs, I don't use it for anything other than playing AVI/MPG/WMV and DVDs thought the MCE interface. I'm planning to whack amiga & c64 emulators on there so I might make a few changes to improve useability there
Vista has been tweaked to hell and back, stripped of nonessential services, thumbnails & document previews turned off, UAC disabled, modded gpu drivers etc. I've effectively been trying to turn it into a gaming rig since my main gaming rig died

OSX Leopard, soon to be Snow Leopard, on my laptop as it is Apple model. Everything else, desktop, server and htpc are running Linux. If it is not UNIX compatible, it is not worth using. :P Got wXP on dual boot though, just for games.