This is a long shot, and I'm guessing that if you turn on your box, and don't choose another os, it'll boot to linux.
Boot to windows, then go to Start > Control Panel > System. Click on the advanced tab, then in the 'Start up and Recovery' section, click 'Settings'. Uncheck the box under 'System Failure' that says 'Automatically Restart'
This is the default option on XP and 2000 I believe. If this does the trick, then you've got to figure out what's making your system fail, as it were.