There was a problem with Avast and Vista (SP1 or less) that sounds a bit like the message you've been seeing (on re-reading the OP).
Basically the computer fails validation after an Avast-related event. Common factors (from what I've seen on the thread at the Avast forum) are that it only affected Vista computers, and most (if not all) of those affected were not running the latest Vista service pack.
The latest program update of Avast addresses this compatibility issue. It is 6.0.1203. (Right click the tray icon for avast and select "about" and it will give your current version. Same again and select "update>program" to update it. Restart required.)
Avast thread
here.
Latest forum update info
here.