continuous restarting after windows boots!!

[will h4x 4 food]

ok my friend came over with his computer and no psu cause his fried so i put my 500watt 1 in there. after we hooked everything up we started it up and we get to the desktop but about 5 seconds later it freezes and restarts.

SPECS
pentium 4 3ghz
1 gig of corsair xms
40 gig hdd
asus p4p800 deluxe