Hi, i had this problem yesterday too, i juts installed a new widget for windows 7 and i got locked in the SafeZone browser, got me a little worried at 1 point but i ended up having to go into safe mode (press F8 on or somthing on startup) then uninstall avast through control panel, then i could restart my computer like normal and uninstalled the widget. After re-installing Avast i havnt got that problem although my opinion of avast has dropped significantly. Hope this is of some help. Itl at least get your pc working again even if you didnt install anything before hand.
Giles