You can try an avast Repair, but I suspect that won't work so a clean reinstall would probably be needed.
- 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 Change/Remove button and select Repair, click next and follow.
You may need to reboot after the repair.
If that doesn't resolve it:
- Download the latest AIS version of avast, 9.0.2008
http://files.avast.com/iavs5x/avast_internet_security_setup.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, avastclear.exe
find it here and save it to your HDD (it has uninstall tools for all versions).
- 1. Now uninstall avast! - using add remove programs, if you can't do that start from the next step, reboot.
- 2. run the avast! Uninstall Utility, it will request it be run from safe mode (and will handle the boot into safe mode), once complete reboot into normal mode.
- 3. install the latest version, reboot.
Note: save the copy of avastclear.exe so it can be reused as some users are reporting that after running it, it is self-deleting.