Well, with all stuff suggested, I had pretty much an identical experience to Kopacha, and had tried almost the exact same things (bar trying new memory). On top of that list I also did a virgin install of XP Pro, XP2'd, patched and nothing else at all. This install doesn't connect to the net, so no AV, Anti-Spyware, no extra software of any description. Didn't help.
HOWEVER I have now completed the game
after making one change.
Someone suggested that changing the CAS latency in the BIOS helped - I changed mine from 2.5 to 3 (the lowest performance setting, lower even than the BIOS default 'safe' setting), and after that I could play for up to half an hour without a crash. So I got into the habit of playing for about 15mins, quitting to the desktop, restarting the game etc etc and I have now been able to finish. The last level didn't crash at all, and I was so engrossed I forgot to quit out.
Hope this helps some of you. Perhaps if the HumanHead peeps are reading this, this might help you pinpoint where the problems lies.