That's simple enough, all you need to do is go into Control Panel, and then into the System area (under the Maintance and Performance area, depends if your in Classic View). Within the System window, open up the Advance tab, look under the Startup and Recovery portion and go into it's settings by selecting the Settings button. Now look under the System Failure section, and you should see the option "Automatically Restart"...it should be checked already by default. Simply uncheck that setting, and your done!
And behold the glory of BSODs! 😀
Hope that helps.