Does it do this when you run another browser, such as FireFox or Opera? My guess is that you've downloaded a Windows or IE7 update that is mucking things up. I rarely use IE but nothing sticks for me.
People are probably going to give you a lot of crap for using IE7 and having a Celeron, too. But since this never used to happen it's probably not your processor's fault. This has got to be a software thing.