Try This .....
Modify auto-reboot setting
Since Microsoft has worked so hard to make this version "the most stable ever," then this tweak is not needed. However, I am fairly sure that they have not perfected millions of lines of code in less than a year. Below you will discover how to turn the auto reboot feature on and off. This allows your computer to instantly reboot upon a system fault. (The blue screen)
Start Regedit. (start/run/type "regedit" click ok)
Navigate to HKEY_LOCAL_MACHINE > SYSTEM > CurrentControlSet > Control > CrashControl
Select AutoReboot from the list on the right.
Right on it and select Modify.
Change the value to 0 to disable and 1 to enable.
Reboot your computer
My putter was re-starting for no reason too... found out it was a blue screen error....... Did the fix and XP let me see the error so I could fix the problem !!!