¿ Which Windows, Spk, bit do you use ?
¿ Any other security program before you installed Avast! and now ?
Repair Avast!:
With XP
Start > Control panel > Add & Remove programs > select avast! and click " Change/Remove ". Look for " Repair " and click it.
With Vista or win7
Start > Control Panel > Programs > Programs & Features > uninstall a program > select avast! and click " Uninstall/Change ". Look for " Repair "
Reboot when finished.