It is could be XP 'blue screening' on you - but in XP the display of the blue screen is disabled, and insted the system just reboots on what would otherwise have been a BSOD.
To enable blue screens (and disable the auto restart so you can actually see the error messages) do the following:
# Go to the Control Panel, and select 'System'
# Click on the 'Advanced' tab
# Under the Startup and Recovery section, click 'Settings'
# Under System Failure un-check "Automatically restart"
Then, try triggering the problem again, and see if you get a blue screen insted, note down the error message. That should enable you to identify where the problem is.
Out of intrest, what configuration is your RAM in? And also, how is your GFX card set up, there is nothing like fastwrite enabled, is there (also, what mode is the BIOS running in? have you tried dropping the bios to safe mode or somesuch, in case its BIOS settings that are causing the problem?)?