What version did it have ? Did you updgrade cleanly or installed over it using Avast! UI ?

If the old Avast! was like version 5 or 6 and you upgrade to 8, it was better to do a clean install. Now try to repair Avast!

Start > Control panel > Add & Remove programs > select Avast! and click " Change/Remove ". Look for " Repair " then Next. Reboot when finished