The possible reasons for your computer freezing up are so many and so varied that I’m not sure I could think of them all. It could be a hardware failure or software, and narrowing them down remotely is very difficult to do.
First, do you get an error message, a blue screen with lines of text on it?
Assuming you are using Windows XP, go to Control Panel, Administrative Tools and look in Event Viewer and see if there is any information related to the crashes.
How old is your computer? Have you opened it up and cleaned out the dust lately? Overheating is one typical cause for crashes.
If it is fairly old, you might also examine the capacitors on the motherboard to see if any are leaking or bulging. Bad caps can cause all kinds of problems with stability.