I had a similar problem except I updated to the newest Avast and everything was still working perfectly. Then Windows Update installed a few critical updates and everything started going haywire. Almost all programs wouldn't run including IE8 and 9 beta, Chrome, Firefox, control panel wouldn't open, etc. I tried system restore many times going back to before the Windows updates and that worked temporarily, but every time I rebooted, the Windows updates reinstalled and everything got screwed up again. I finally did a complete system restore and started reinstalling all software, etc. I got all the Windows updates installed, and everything was working perfectly. Then I installed the latest version of Avast, and I started having all the same problems like before. I uninstalled Avast and installed Microsoft Security Essentials until this problem gets worked out, and everything is running fine now. There is definitely a problem with the new Avast version and the latest Windows security update files from several days ago.