Well, you know my well argued opinion on mine
Vista.
I do use XP quite often, specifically in school, and on our other home computers...
My opinion is:
If your computer came with Vista, stick with Vista, if it came with XP, stick with XP. (And I think quite a few people here share that opinion)
If it werent for driver issues, and finding free antivirus, I would use Windows Server 2003 on my primary computer... extremely stable, free to students, great OS.
And I use Ubuntu Linux 8.04 Hardy Heron (no GUI) for my server...