Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot.
It would probably be best to download the latest version of avast 4.7.817 and save it to your HDD, somewhere you can find it again. Use that when you reinstall.