If you have a chance before the computer freezes, jave a look at Administative Tools>Event Viewer (particularly System and Application) and look for any error messages that might give you a clue about the problem. Right-clicking on any item and selecting properties brings up more information. there's sometiimes a link to check out.
Have you done a hard disk check?
Another suggestion is to download a Linux LiveCD and run it: if it runs smoothly, you can say that the problem is with Windows or the HD, because a LiveCD doesn't touch the HD.
If Linux has similar problems, the LiveCD has a memory check option: use this to test out your memory.
That's about all I can think of!