What programs are running when it happens? Any particular thing you can think of that triggers freezing? Any new software installed that could cause it?
Do a scandisk, defrag, update spyware / firewalls / antivirus and scan for viruses, spyware etc, see if that helps.
Also use task manager to close all background programs (other than windows!) and see if that makes any difference.
Failing that, you might be looking at backing up and formatting....