From
http://windows.about.com/library/tips/bltip601.htm
Nothing can be more frustrating than when your computer won't boot properly. Whether you've made a recent change, installed a bad driver, or whatever, sometimes the darn thing just won't come back up right after powering down. There is a hidden boot menu in Windows XP that has an option that can help. When the Operating System first starts (after the power on self test), press the F8 key on your keyboard. This will display a menu on a black background, which has an option called "Safe Mode." Actually there are a couple different Safe Mode options, depending on whether you want to use the network, or just a command prompt. Choosing Safe Mode will allow you to at least boot up, so that you can uninstall a bad driver, or change problematic system settings, etc