Problem w/computer shutting down

Ever since I wiped my hard drive clean in order to
reinstall Windows XP, my computer will shut down on its
own while I am in the middle of working on something -- I
could be online or not, in the middle of a MS Works
document, whatever. It will not restart by pushing the
ON button. I have to turn off the power surger, wait a
minute or so, turn on the power surger, and then push the
ON button in order to get the computer to come back on.
I've tried about everything, including plugging the
computer into different outlets -- except to buy a new
power surger. Any suggestions? Thanks!