First of all, if you have a Via chipset, don't update it's drivers. The ones supplied with XP are extremely stable, and have yet to be outdone. As far as reboots, right click on My Computer and choose Properties. Click the Advanced Tab and then the settings button under Startup and Recovery. Unselect Automatically Restart from the window and close. That should fix your problem. Keep in mind that the system restarts due to a system failure. This could range anywhere to a missing txt file to a corrupted wav to a virus.... so it's not always a crucial situation. Write back with results.
P.S: Have you gotten all the crucial updates(and drivers) from
www.windowsupdate.com? Might be a good idea