What antivirus did you have before installing avast ?
Have you removed it ?
Using ad/remove program, and reboot ?
Have you run the uninstall tool for that program and reboot ?
http://uninstallers.blogspot.com/Have you tried avast repair ?
For a repair of avast. Windows, Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button from the pop-up window, scroll down to Repair, click next and follow.