There has been a major rash of serious FP's from Avast today.
Updating to the latest VPS database fixes the problem.
This is now fixed. It was extremely serious. More info
here.
Anyone who has had programs quarantined or not working as a result is advised to rescan each infected file from within the virus chest, and if it scans clean, to restore it.
Some people have lost their OS over this one, I'm afraid. Usually those who responded with an "OK" to a boot scan prompt, or those that deleted rather than quarantining files. In either case, important files required to boot the system have been deleted or quarantined. The chest can't be accessed unless Windows is up and running.