If your system shuts down or crashed you may find yourself in an irritating position where the system constantly restarts in a loop while attempting to start Windows.
This possibly caused by the registry becoming corrupt as a result of an application writing to the registry at the time of a system crash. The next time the computer start and tries to read the registry the system fails and has to reboot.
To Fix the Problem
• When the system reboots, shortly after the initial beep press F8
• choose "Boot from last known working configuration"
Steps To stop constantly rebooting During Boot up
1. Click Start
2. Settings (Skip on Windows XP)
3. Control Panel
4. System (May need to click "Switch to classic")
5. Advanced Tab
6. Press Settings button
7. Uncheck "Automatically restart"
This will stop Windows from rebooting even when it encounters an error during the boot up procedure.