Windows 7 Ulimate 32 Bit Crashing Frequently On IE And Firefox?

I recently had a major problem and had to do a system restore which seemed to work. But now (later the same day), I have multiple system freeze-ups, first my mouse pointer changes back to the default, then programs lock up (Internet Explorer and Firefox both), then Word freezes and then the entire system becomes totally unresponsive. Even CTRL-ALT-DEL won't work. My only choice then is to do a hard shut down. It always restarts but within an hour, I go through the same gradual freeze-up and then lock-up. I don't know if this means anything, but each time the computer has frozen, I have been working on Blackboard. When it happened in Firefox, I switched to IE but now it's happened there, too.