Random Shutdowns

[Cook1]

Working on an older PC of my mothers. Specs off the top of my head are...

AMD Athlon 1900+
PC Chips S-A board
512MB PC2100
Shared Video card which makes the total system RAM 480MB
40GB IDE Hard Drive
Thermaltake Purepower 420W

This was happening about two months ago, so I installed that TT PSU new from the Egg and did a clean install of Windows. Seemed to have fixed the issue but now it's back again. I've run diagnostics on the Hard Drive, and it's fine, running memtest as we speak. What else could I look at?

I mean, I've dealt with thus far:
PSU
Hard Drive
OS/Apps (from clean install)
RAM- Doing this as we speak

 

[hardcandy2]

Are any of the Auto shutdown features enabled in Display-->Screen Saver-->Power Management?

 

[Cook1]

Hard Disks and System Standby are set to "Never"
Turn Monitor off is set to "20 Minutes"

So that's not an issue.

 

[Cook1]

Well had an error within 15 minutes, I would imagine this would be a leading culpurit.

 

[hurtstotalktoyou]

I was having a similar problem with a PC Chips board. It turns out it was a result of a bad hardware driver (not bad hardware). If you've updated any of your hardware drivers via Microsoft Update, try rolling them back.

 

[DotheDamnTHing]

dust? bad HSF? loose screws rolling around that may cause a short? bad RAM?

 

[Cook1]

Cleaned it, reseated the HSF, nothing is loose and I haven't installed any new drivers for it.