Hi RachelWiz, welcome to Bleeping Computer!
The broken fan may very well be the problem. If not, try to remember what program(s) you had running each time the computer crashed. If there is a particular program that's always there, it may be the program's fault. Same with hardware devices.
In the mean time, go to Control Panel -> System and click on the "Advanced" tab. Click the "Settings" button under "Startup and Recovery" and remove the check mark next to "Automatically Restart". Then click "Ok". THis will prevent your computer from rebooting if it crashes and will instead display a scary looking blue screen wth an error message in white text. Copy that error message down (be sure you get everything) and post it here. We may be able to track down the problem from the message.
Edited by Amazing Andrew, 30 March 2007 - 06:48 PM.