What is your OS ?
What version of avast are you using Home or Pro ?
What is the version number, the latest is 4.8.1229 ?
If it isn't that version then perhaps it would be best to do a clean reinstall.
Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
1. Now uninstall (using add remove programs, if you can't do that start from the next step), reboot.
2. run the avast! Uninstall Utility, reboot.
3. install the latest version, reboot.