Hi
Faulty OS,
Faulty PSU
Faulty Board.
Check the board capacitors for any that look bubbled on the top and rusty marks on them, sure sign they are failing.
Swap out the PSU.
Press F8 after the PC Post screen and before the XP logo and from the menu choose safemode and see if it restarts in that. If it doesn't its definately an OS fault, if it does refer to above.
Remove all devices apart from one stick of memory and the VGA card and the bootable drive, now see if it does it, if it doesnt then one of the items removed may be responsible, if it does refer to above
Just some suggestions...
Regards