Been playing with a few different operating systems lately.
Recently i also started working in a computer shop giving me access to crazy amounts of gear .
Windows 7...... wow what can i say. Amazing on just about any system.
Currently only in RC form, will need to pay some time this summer.
(Ran smooth (only scored 1.1 tho ) on AMD 2800+, 512MB RAM DDR400, 64MB Onboard video! Even aero!)
Windows Vista.... recent Service Pack has fixed alot of issues, still ram hungry.
(Still chews 800MB of RAM sitting still)
If unsure stick with XP for now