what AV program did you have before installing avast ?
have you removed it ?
did you run a removal tool to clear out any leftovers ?
Run and reboot:
Uninstallers for Security Software
http://thewebatom.net/uninstallers/security-software/if still problems try avast repair....or reinstall
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.
