Win7 64bit on my gaming rig, XP 32bit on my laptop, XP 32bit on my work rig, Win98 SE (VM) also on my work rig.
No tweaking, no pimping, no problems...

Win XP Home on my main rig.
Win XP Home dual booted with Linux on my secondary rig.
I can't recall offhand all the tweaks I've done with them, but I have turned off some services.
Building a new rig this fall and will be installing Win 7 64 bit on that one.

Windows XP Home on my laptop, and Windows Vista Ultimate x64 at home, but I have UAC off because it's so annoying and I know better than to install malicious code anyway. I'm probably upgrading to Windows 7 Ultimate 64-bit when it's out though.

Windows 7 RC x86. Love the new taskbar look (no need for docks), jump lists, aero snap, and the ability to listen to my Cowon S9 through line in. The new Wallpaper features are nice too. My only gripe with it is the same I had with Vista where I couldn't have all my directories show icons in list view via folder options like I did with XP.

Windows Vista Ultimate 64 bit on my desktop, and Vista Home Premium 32 bit on my laptop. Until recently, I ran a dual-boot with XP Pro, but I removed it, as it was rarely used.

I use Windows XP Home and after some time I'd like to switch to Win 7.

I use Vista Ultimate (32Bit). I usually go through MSConfig and Services.msc to disable and change certain things, like UAC and various services I've got no use for. I also use CleanMem, CCleaner and MyDefrag to keep it running at top speed, and RivaTuner to control the fan speed on my graphics card. Apart from that, there is some theme customization and such, and a bunch of background applications mostly related to gaming, like Steam, Hamachi and Teamspeak.
EDIT:
I'm also dual booting with Ubuntu, though right now it's just installed because I had some boot problems not long ago.

Arch Linux! Woohoo!
<runs around in his penguin-print underwear>
How's Gentoo? That's the one where you have to compile everything yourself, right?
How's that working out for you?
Anyway, I have a dual boot vista/arch linux. Vista has as many fluff disabled as possible, and I'm reasonably content with it. I plan to look into W7 though.
Arch Linux is pretty simple. Openbox window manager with a great tiling script and dmenu to run most programs. I do most things via the console, actually.
Also, I don't really have linux themed underwear. The most geeky thing I have is my VERY AWESOME GOG shirt.

Sadly, I don't find any use for a Linux distro... otherwise I would use a linux and have some geek points by simply having one (not that Ubuntu...).

To be honest, there aren't many reasons to have one.
I can think of:
1. it's free, but on the other hand, most pcs come with windows installed anyway.
2. you are a programmer. Linux is (at least for me) a much more natural environment to work in.
3. You like tinkering and understanding how and why things (don't) work.
But for the average end user, Windows is a better and more convenient option. Or the Mac OS, I suppose. But I really, really dislike Macs =P

win xp pro.
i absolutely hate vista. i took years to make everything work on and for the XP. Now it finally does they come up with a real joke of an OS.
games tend to have much more problems running on vista machines.
Inexplainable errors [/rant]
i use the vista look on my xp version , that's the only think i do like about vista : sidebar with all the gadgets and topbar for easy access.
XP is tweaked to boot under 20secs
so i'll skip the whole vista story and go for W7 i think.

7 just looks like the version of vista that was finished.

VIsta 32 Ultimate.
Gotta say, I dissed vista a lot and saw no reason to upgrade, but it's a BIG step forward from XP. Boots WAY faster. Starts applications WAY faster. Loads of really nice little tweaks to the interface (custom columns, search/filter in a folder, start menu works as a great app launcher, search, etc..)
While it's not a total revolution, they've fixed so many of the tiny little things that used to bug me about vista (eg: renaming a file no longer selects the extension). And the new windows update is great.
This has had the advantage that i've been able to do without loads of apps i used to have installed. (startupdelayer, locate32, find&run robot, freecomander - couldn't live without them on XP, don't need them on vista).
I will say that Ultimate is a total rip-off though. If i hadn't needed the multi-lingual support (which should be in all versions imho) then it'd have been a total waste of cash.
UAC didn't annoy me at all. Maybe it's cos i got vista after SP2, was it improved? I like having it, but i rarely see it.

Windows XP Professional Service Pack 3.

Definitely agree with that, I was fortunate enough to get it as an OEM with some RAM I bought so it wasn't as MUCH of a ripoff but it definitely feels like the "Lets see what the suckers will pay for" SKU. I mostly got ultimate on the theory that I might have to support vista business, typically it becomes obsolescent as I get closer to employment. Still there's all the fun of the migration to 7 to plan I guess