Many times, previous anti-virus leftovers prevent a proper anti-virus installation. Under these circumstances, you may be instructed to do a clean installation.
The 1st step is to download the avast! Removal tool “aswclear.exe”
http://files.avast.com/iavs9x/avastclear.exe and save on your desktop.
Right click the avast! tray icon, and open the avast! User interface.
Go to Settings, Troubleshooting, and clear the box “Enable avast! self-defense module”
You must acknowledge “Yes” to fully disable the avast self-defense module:
2nd step is to remove the avast! anti-virus from add/remove programs or Programs and features.
Now, you may run the avast! removal tool “aswclear” saved on the desktop, and you do not have to run it in safe mode, even though it tell you so, due to the avast! self-defense module already disabled.
Select “No” and proceed:
On this screen, you will have to choose the version of avast! that we just un-installed. NOTE: If you did NOT change the default location during installation, then you may leave / ignore the top 2 locations as “unchanged”
Reboot the system, and now you are “avast! clean” and can now install the new version of avast! anti-virus.
Hope this helps you out!