These are totally normal question, for XP I could help you right away, for Vista I will do some recon/research (maybe someone who know "manage", "msconfig" and "performance settings" in Vista could help us here
)
btw 3 GIGs of ram should be sufficient...
don't you have your pc for longer time (check the heat pipes of your video card, clean it from dust) and check if you don't have trojan horse/ virus/ malware/ spyware, what could take your CPU/ HDD performance