Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and select Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
You may need to reboot after the repair.
I suspect that you will get the same error for the repair also, if so:
- avast! Virus Definitions Update - Manual Download
VPS Update (virus definitions, vpsupd.exe, approx 78.4 MB). - 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. The one you want is the one for avast versions 5/6/7 it is the same for 8.