Another smiley on boot

I've read the thread http://forums.majorgeeks.com/showthread.php?p=1112358 and it seems to be the same problem...I see a hash followed by a smiley face occasionally when I boot. But I can't get the computer to run for more than 40 or 50 seconds before it reboots by itself.

My first thoughts were overheating. The fans seemed fine, and there wasn't much dust or fluff. I removed the heatsink and applied new paste (the old paste was dried out), but no improvement.

I got a few error messages saying I'd overclocked (underclocked in fact - multiplier at 10 instead of 10.5) or that memory had failed POST, but the error would be different on the next boot - or there'd be no error.

I use NOD32 for virus control, ZoneAlarm Extreme for firewall (with virus protection disabled), and Malwarebytes for adware/spyware. (All paid versions)

I can't follow any of the advice on the other thread (http://forums.majorgeeks.com/showthread.php?p=1112358) because I can't start the computer. Is this in the BIOS rather than the HDD? Would flashing the BIOS be likely to fix it?

Thanks,
Russell