Do you have any other security software installed?

If not, then it’s probably best that you use the avast uninstall utility for all versions of avast that you have used to make certain all old program files are removed.

First, download to your desktop the avast 2014 free version from here → http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe

Then, follow the instructions here → http://www.avast.com/en-us/faq.php?article=AVKB10#idt_02

After this, reinstall avast.