Windows

Go to Start> control panel>system>Advanced and unchech box labeled Automatically restart. Then see if there are any errors when you log off.
Balbir

The auto reboot would option is good if your system is crashing consistently.. Here he states its only when the user logs off..

My question, does this happen for all users or only one?

If its only one, I would save their data to a new folder, logon as an admin user and delete the profile for the problem user..Then recreate if needed.

If you system in question only has one user, then I would look for virus activity. when you hit the logout button it should launch the rundll system logout sequence, not the reboot command.

Please keep me updated with this issue, interesting...:)

Good Luck

This would note that your system is crashing check the device manager for any yellow exclamations beside the hardware

Make sure that video card is installed properly.

Check the system for any IRQ or Hardware conflicts,which will cause the system to reboot,try basic troubleshooting,because this type of problem can be caused by any number of things.