I usually use these guys:
Watch the first 15 seconds and then around 1:05 min. Solves all my problems
But seriously, sounds like a HW problem as described above. Apart from power supply I would also check the memory - faulty chip could cause what you described: you should be able to download some memory test tool, also try pulling the memory out and back in, sometimes it can get loose and the pins don;t have a good contact.
Other suspect could be a corrupt sector on the hard disk, especially if it's where the swap file is. I don't think this is the case because you should see a message saying windows recovered from serious error, or something like that. But it is always good idea to check the system log (right mouse click on my computer icon, select manage, system tools, logs...).
Also check inside the computer, sometimes they are full of dust, which is never good.
You can also try disconnecting (if you know your way around the PC) any other cards and devices you have, especially any PCI, SCSI, PCMCIA and other cards. They could cause the system crash too. (make sure you disconnect the power and earth yourself back to the cage - use a piece of wire, you don't want the static electricity ruin the chips).
Hope this helps.
