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 may need to reboot after the repair.
If this doesn’t work you may need to download the complete virus database, the repair function should say if that is required.
Did you try the repair first, that may be enough, we can cross other bridges as we come to them. Of course you can take the reinstall route, but generally we suggest the lessor hassle of the options.
Avast! Manual Virus Definitions Update Download - avast! Virus Definitions Update - Manual Download ensure that you select the correct virus definitions (for avast 5, vpsupd.exe, approx 31MB). - Save the file to your hard disk, locate where you saved it and double click it and that should carry out the virus definitions update.
I had the same problem. Uninstalled and reinstalled. Fixed that problem as well as some other issues. Slow startup times and other minor things. It is not hard to do, but I did have some issues removing AVAST. It was like it was fighting itself and making start up and loading websites vey slow.