The problem is Firefox 4. I have two PC's with one XP OS, the other Win7. Only problem on XP is the search drop down options do not work when clicked on. I too have nvidia drivers on this PC. However on the Win7 OS, with MSE and MS firewall, the PC freezes regularly and is unresponsive for several minutes. Mozilla recommend running browser in safe mode after turning off all add ons and then reintroducing one by one until the culprit is discovered!! People have tried doing as suggested without finding a solution. You have probably noticed there are numerous topics about this problem in Google. What is clear is that people have various different set ups but that the one connecting factor is the change to Firefox 4. Instead of telling us what the problem might be with our set ups, and asking us to waste substantial time on work a rounds, perhaps Mozilla should look at Firefox 4 and tell us why it is causing the problems?