Ordinarily I would suggest a Repair of avast, add remove programs, avast, uninstall/change, when the new window opens select Repair, Next and OK your way out, reboot.
This still may be the easiest way forward, but avast 8 has just been released
http://forum.avast.com/index.php?topic=116245.0. If the above resolves it you can do a manual program update from the avastUI, Maintenance section.
