Ron:
One of the first things you are going to want to do is look at the event log that windows XP keeps.
Right click on "My Computer" select "Manage" and you'll get the general overview MMC panel.
From that, select "Event Viewer" First, look at your system events.
You're looking for red "stop" or "fail" events first. Look to see if you have a pattern, particularly service failure, disk warnings, etc. This will give you a clue as to what stopped before you generated a BSOD.
After you've done that, you can try to check your RAM with several of the free online utilities. The most reliable one (IMHO) is Memtest86, a free utility you can download here:
http://www.memtest.org/
Memtest is free and can be downloaded in a bootable ISO format or floppies.
Before you get to that, though, you would be well advised to check your log. If you did have a memory error, you would normally be crashing far more often then "rarely".
You might also check general maintenance issues.. is the CPU properly cooled? (fan running correctly, not overly dusty, etc.?) video card fan working properly? Good ventilation? Is the power supply ready to uphold the load you're asking of it? Is there a rogue application (noticable via event viewer) that routinely crashes?
Anyway, good luck with whittling down your choices. You can always run a Belarc or Everest (
http://www.belarc.com/ or
http://www.betanews.com for the latest version of everest) to get a detailed spec of your PC to find out how you're doing on some of those issues
