I was having same kind of problem in windows xp (i wanted to play the witcher). Problem was caused by X-FI when i installed X-Fi driver and restarted my pc would just shutdown after reboot (and every time). Only thing that helped was install VISTA drivers for xp (but it still couple times shutdown when playing the witcher) but with xp driver i even can't get pc to log on windows
(appereantly something to do with IRQ sharing). So try with X-FI uninstalled will it do the samething or try install vista drivers.