The default setting for any version of Windows is to restart the system upon boot failure. This can be stopped by pressing the F8 key after POST. You'll then be taken to an advanced boot menu. In this menu you can disable the resart on boot failure option. Doing this should then restart the system and halt at what we like to call the Blue Screen Of Death. On this screen you should see an error message with a brief description of what Windows thinks the problem may be related to.
If that doesn't help, click on the link in my signature and follow all of the troubleshooting steps. Don't skip anything just because you don't feel it is relavent.