We have discussed this topic many times, but I continue to hear from people who forget this simple troubleshooting step. When a problem arises that seems insurmountable or you can't move your mouse or type on your keyboard, shut the computer off at the switch.
There are two ways to shut your computer off when a problem occurs...actually three ways.
Once you shut your computer off this way, let it sit in the off position for a few minutes before turning it on again. If you are using Windows 95/98/Me, don't worry if it barks at you a little for 'not properly shutting down'. With any computer, this isn't the preferred method of shutting off the computer, but in a pinch it usually gets you out of a jam.
Links:
[1] https://www.paypal.com/cgi-bin/webscr?cmd=_xclick&business=rick@yourpcpartner.com&item_name=Buy
[2] http://www.helpmerick.com/node/1110
[3] http://www.helpmerick.com/node/1103