Have a PC here that will just switch itself off at random, by switch itself off its like someone pulling the power chord - no BSOD, no warnings, nothing. It is totally random and can go days or a week without doing it or may happen three times in the same day.

I've run memtest on it with no errors, CPU temp is around 46 at idle and up to 60 at full load, PSU is a heavier duty one (680w), hard disk has been scanned for nasties using several software solutions (Malwarebytes, Spybot, A-squared, Avast). Have done all the usual checks.

It also is very slow at startup and boot, I've turned off everything non-essential and streamlined the process but to no avail.

Specs as follows:

Win XP64 Pro SP2
AMD X2 5600+
4GB DDR2 Kingston 6400
ASUS M3A78 board
Nvidia 9800 GT 512 Graphics
Seagate SATA 300GB HDD

Any suggestions on what else to check or any full diagnostics software recommendations?