I would suggest going to control panel - system - advanced system settings - advanced - click settings next to startup and recovery (3rd button down), then untick automatically restart. This will mean instead of automatic reboots you will be presented with a BSOD. That BSOD may or may not give you useful info pointing to what the cause is.
on that same page also check write an event to the system log is ticked.
Edited by Chrysalis (Wed 26-Jan-11 13:13:59)