You could 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.
However, I would suggest you download the latest version 4.7.1001 (don't know where you got 4.6 from), uninstall the old version, reboot and install the latest version. Download from here,
http://www.avast.com/eng/programs.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.