what antivirus did you have before avast?
have you uninstalled it? (only install one AV)
have you removed any leftover from that program with a uninstall tool ?
you can find one here http://uninstallers.blogspot.com/
run avast repair and reboot
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.