From the web site:
http://www.gimp.org/windows/
Requirements (in part)
GIMP should run on any NT-based version of Windows (NT4, 2000, XP or Vista).
(Oct 2008)
Windows explorer is the process which allows you to view your desktop, icons, browse folders. You can find it by opening task manager and locate explorer.exe
I would say Windows 7. Normally i would say it's what suits you but in this case, know for a fact that Windows 7 is much less frustrating than Vista. It doesn't crash (as much), its much faster,...