random shutdowns

My friend has/had a problem with his machine shutting down
Running XP pro on a 1.4 Gig AMD
w/ 512 MB DDR 2100, W/ a broadband connection
Initially I suggested it might be the power supply.
That tested fine. Someone suggested it might be a virus.
unfortunately at this time he had already tried two
other "fixes" He had flashed the Bios because the machine was seeing
a 128Mb video card as 64MB and his old 64 as 128.
he also tried to reload via upgrade.
I had him FDISK the Master boot record and do a fresh install of XP
Now he says that he has downloaded SP1 from Windows update and during install it took 3 hours.
I have suggested 2 possabilites here
1.) that IF he had a virus it may have infected the files he downloaded
and flashed the bios with and,
2.) That he has other partitions which he did not blow away which may
have reinfected
his new install. I have suggested he do an online V scan.
Have we missed something? and does anyone know how to either scan
Bios for Virus, verify or fix CMOS if it is either infected or simply a buggy
install?

 

turn off the auto reboot to get bsod error message pointing in the direction of the fault,and post it

 

Thanks Dai!
but the machine is not rebooting. It is shutting down

 

check all the fans are turning
check the heatsink is seated
are there any beeps
check the temps in the bios