All bets are off on the PC front. I'm now pretty convinced it's not a hardware problem. It's an Opera problem. I got my PC back up and running last night, immediately ejected my Black Hawk Down DVD from the DVD drive, and uninstalled Opera. Lo and behold - the PC ran stably all night.
To start this story at the beginning, we need to go back a few weeks. When I downloaded the new dial-up software for my BT Internet account, it inexplicably broke my Internet Explorer install, such that it stopped autoloading images, an no amount of reinstalling IE would change things. So, I decided that I'd grab Opera off the PC Format disks and give it a try, as I'd heard vaguely good things about it, and it's "Open Source", that paragon of anti-capitalist stick-it-to-Bill software.
No sooner had I installed Opera, and given it a try on the 'net, I started having my initial Black Screen Of Oblivion problems. So, I did all the right things, under the impression it was a hardware problem, reset my overclock, and on a whim, I uninstalled Opera. I did a Scandisk which fixed a problem with the boot sector of my hard drive, and everything was fine again, for about a fortnight.
At the weekend, I wanted to look at a few flight prices to get a ballpark figure for my summer holiday, so I tentatively reinstall Opera and have a poke around Expedia for my flight details. I leave the machine on, play a bit of X-Wing Alliance and Freespace 2, watch Black Hawk Down, and call it a night. Sunday morning, I turn the machine back on - boof! - BSOO. Monday night, I turn the machine on again, skip the automatic Scandisk, uninstall Opera, Scandisk to fix the boot sector problem on the hard drive, and reboot. And it's fine, even coping with a 4 hour session of Star Wars: Supremacy (still the finest game in existence). If it's fine tonight I know that I don't have to spend £300 on a replacement motherboard, CPU and memory. I reckon my PC has taken a dislike to the Internet in general and Opera in particular, because every time I use it, it appears to bugger up my boot sector and give me BSOO's.
So it would appear that the moral of the story is either a) don't use Open Source software, b) don't use the Internet, or c) both a) & b).