Which version of avast?
Was it a trial version or had you registered it.
It is almost certain that setting the system date to Nov 2005 has triggered the problem.
Probably the easiest thing to do is, download the latest version, download the 'Avast Uninstall Utility,' (
find it here).
Once you have done that, uninstall avast using the windows ad remove programs and reboot. Run the Avast Uninstall Utility to ensure that all remnants of avast are gone, and reboot.
Ensure that the system date is correct, now you can install avast and past in your old key. Let me know how you get on.