Hi eros211, and welcome to BleepingComputer.
When it starts press F8 and hold it, a black screen will eventually load with options. Use the up and down keys to navigate to Safe Mode, and then click on Enter. When Safe Mode opens go to Start> right click on My Computer> Properties> Advanced> Startup and Recovery.
Under startup and Recovery click on Settings, uncheck the box in Automatically restart under System failure.
When you restart your computer rather than it restarting now it should show an error message. Please post the exact error message here.
Would you please post the make and model of your computer?
If this is a desktop, when was the last time you cleaned out the inside of the case? Overheating can cause this to happen as well.
Edited by dc3, 25 August 2010 - 11:27 AM.