Blue Screen of Death. If you disable the auto restart, hopefully the machine will stop on a Blue Screen of Death (BSOD). Record and post back with the "exact" error message text and/or numbers plus any offending software module it might name.
Use the <F8> function key during system startup to access the Windows Advanced Options Menu. Tap the <F8> key once or twice per second as you startup the computer (or immediately after POST completes and screen flashes black prior to displaying Windows XP logo with its processing bar display).
From the Windows Advanced Options menu, use the up/down arrow keys to select the "Disable automatic restart on system failure" and hit <Enter>.
This item became available with the release of Service Pack 2. It is not on the Windows Advanced Options menu of Service Pack 1.