Your machine could be rebooting for any number of reasons.
1. Go into safe mode.
2. Go to Start|Control Panel
3. Double click on the System icon
4. Click on the Advanced tab
5. In the Startup and Recovery section, click on the Settings button
6. In the System failure section, clear the checkbox next to automatically restart and close out this window.
7. Reboot your machine.
You should now get a blue screen of death with some error information. Get the top line of info (for example, it maybe IRQ_NOT_EQUAL.... etc.) and report back here with the info.