I first had this problem when I installed a new game: rome total war. it installed fine then wen playin the pc just turned itself off. I was going to reinstall windows anyway so I started to do that. booted the machine from the cd rom an started to install, then it turned itself off just b4 the the option to install windows xp. so I ran the hard drive as a slave off another hard drive and the pc worked fine I did a format on the hard drive I needed to install windows. then booted the machine with the clean hard drive and started t install windows but the machine turned off again.
the mobo also has a 24pin power connecter and I'm using a 20pin connecter from the psu. the machine has been running fine like this but am gona change psu soon.
any more info needed just ask.
3.4ghz duel core pentium D
asus p5vd2 mobo
jeantech 400w psu