Persistent BSOD, Dell Dimension, Windows XP SP3

After more than three years without such problems, my PC suddenly crashes with the blue screen of death. It runs a few minutes after each restart, and then it's BSOD every time. Reboot requires unplugging from power and plugging back in.

The first BSOD message was

   A problem has been detected and windows has been shut down to prevent damage to your computer.

   …

   STOP 0x00000008 (0xC0000005, 0xF786A38, 0c00000000)

   IRQL_NOT_LESS_OR_EQUAL

After that happened several times, the error codes started to vary. At this point, the PC crashes before anything useful can be done. It's the same in regular start-up or safe mode or "safe mode command line."

Configuration: Dell Dimension E510, Windows XP SP3, Office 2003;

      Free versions of ZoneAlarm, avast, SuperAntispyware.

There have been no recent hardware or software additions/deletions.

I tried these things without success:

  - Safe mode

  - System restore to a week ago (I don't think the restoration completed, though, and fear an incomplete restoration might've made things worse.)

  - Moved keyboard/mouse from display’s USB ports to PC's

  - Chkdsk

  - Virus scan but it couldn't complete (avast)

What can I do now? (I've read about disabling BIOS memory caching, but how do I do this?)

In particular, is there a way I can decide if it's a hardware failure or software problem? I suspect hardware because the first BSOD was in the middle of normal operation, but after that it happens at startup or shortly afterward.

In the event log at http://sites.google.com/site/zvkmpw/home, the first BSOD is about 10-7-2009, 2:22:00. Notice near there the entries about "A device attached to the system is not functioning" in a network context.