pc shut down w/o notice

hi hope one of you guys can help me out ive got a pc lga775 p4-3.0ghz ,512mb-ddr2 ,80gb-hd ,brand new powersupply, new motherboard,new ram runnig xp pro sp3 just installed. the problem is that it sudenly shutdown & restart without notice after 2 to 3 hour after turn on.iv monitor temp to see if cpu is not heating up but its working fine + i dont tink its a virus problem.by the way can the new ms sp3 cause this kind of problem?
tanx hope some can help.