Geforce 9600 Gso Monitor issues

emm how do i put this, bought a factory built pc a while ago well in feb 2009, and at first i fobbed it off as it only affected a few games and I said it was my monitor in my mind, then I tried it on 2 other monitors , the problem of the monitor shutting off and the only way to return to normal is to force a reboot, i knew it wasn't my monitor then so took it to a friend who is tech savvy, he said to back up the data and install win xp in case of it being a corrupt OS however when i was moving it i found it only has a 250 watt power supply unit and nvidia says the 9600 requires a 400 watt minimum so what's the cause ? OS or Psu please tell me Psu as i don't want to nuke the hard drive