Windows crashes-doesn't allow updates or Norton AV installation

I just recently did a clean install of Windows XP on a new harddrive. In the
process of bringing it up to "speed" with all of the updates, etc. I find
that the computer will simply reset for no apparent reason. I have install
Service Pak 2 and it did a few updates, before I encountered this problem.
It will also not allow me to intall Windows Defender nor Norton AV. While in
Windows Explore, Explore will just quit and ask me to send an error message,
which of course, never gives me any clues at to what to do.

Short of wiping out the hard drive and starting over, does anyone have any
suggestions of what to try. It is a Western Digital 120G HD. I did the WD
diagnostics on it and it is fine. I also did a RAM memory check. I doesn't
seem to be an overheating problem since I have left the machine on for DAYS
and it will run fine, then out of the blue it resets and then runs fine for
several more days.

Any help would be appreciated.

Thanks,
Doc H