Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and choose Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Change/Remove button and choose Repair, click next and follow.
Reboot after the repair.
DJBone