If you are getting random restarts in Windows XP there is a default setting to "restart on failure" and that is quite probably what you are experiencing. There are a couple of things to try.

1. Right click on My Computer and select Manage. Expand the Event Viewer category and look through each subcategory for the red flagged error records. These will probably coincide with your restarts. Double-click on an error record to bring up the details of the error.
   
   
2. Open Control Panel - System - Advanced. In the Startup and Recovery section (bottom) click on the Settings button. Under System Failure there are 3 checkboxes. Click on the "Automatically Restart" box to clear it. While you are in there change the "Write debugging information" item just below it to "Small memory dump (64 KB)". There is no need for the full memory dump unless you are a system programmer or are working with one to diagnose the internal details of the failure.
   
   

The second change should put an end to your restarts. However they will probably be replaced by a Blue Screen Of Death STOP error message and your computer will shut down completely, requiring a manual power off and restart. But the STOP error message will give you specific details as to the cause of the error and that should help you to identify and remedy the cause. Make sure that you write down the details of that error message.