When the blue screen appears, there's usually a stop-code in the top part of the screen:
Code:
Stop 0x0000000A or IRQL_NOT_LESS_OR_EQUAL
or something.
Check
this page to see what these means. Usually it's a device driver problem, but it could be more serious. After you've verified the error, reboot and press F8 whilst booting then start XP in safe mode and disable the device that is causing trouble (in Device Manager).
Good luck. BSOD's are bad, but no reason to start formatting your HD right away!