Just a guess, but if the psu is the oem version, I would take a look at that first. The psu weakens with age, and a failing psu can cause many problems.
You really can't have too much power, only too little.
I'm running a HP Pavillion (751n) with XP Professional (upgraded from XP Home).
If I run any utility (chkdisk, Full Norton Antivirus Scan) or run any game (e.g. Age of Empires) the computer will *snap* and down it goes. No warning, just instant shut down. It even happens when running PC Certify diagnostics from the A: drive.
Nothing of any import shows up in application or system event viewer. Dr. Watson doesn't record anything.
When I start the machine up, I don't get any dire messages (like "the system has recovered from a serious error")
I've updated the SiS board drivers, but still the same.
Anybody have any ideas?