OS: Windows Vista. Despite the UAC being a complete pain in the backside I truly don't mind having to click confirmations that yes indeed I do want to open up that program for the 100th time. I may have been an early adopter but I'm not completely stupid. This is Windows and it is NEW which separate are bad enough when it comes to computers but when put together it morphs into something much worse than the sum of its parts.
Gotta give props though about the user interface still being pretty simple, and the fact that during an install you can now use CDs or USB keychains to load 3rd party drives (thus removing the need for floppy disks forever) brings small little tinge of warmth to my bad empty heart.
Quick benchmarks:
Doom 3 on my old system (an Athlon64 3000+ with a Radeon 9800 Pro) gave me about 32 frames per second at 1024x768 and 2xAA.
The new system gives me 97 frames per second at those same settings. When I cranked the details to ultra high maximum the frames still maintained about 32 frames per second.