if you want to use drastic messures, you can do the following guide which SHOULD let you install avast 7 without uninstalling the corrupted avast.
!!! THIS IS ON YOUR RISK !!!
1. - enter safe mode, delete Avast by removing C:\Program Files\Alwil Software directory (avast wont be able to lock you down, this is because avast isnt running in safe mode).
- after that, go to start > search > search for "avast" and remove any files which come up in the search.
- ok now go to start > search > regedit:
*go to HKEY_USER... > software > remove "Alwil Software" registry directory.
*go to HKEY_MACHINE... > software > remove "Alwil Software" registry directory.
2. go back to windows normal mode, go to c:\program files and remove to recycle bin the "installshield installation information" directory.
download avast 7 installation from the forum, install it (as an administrator) where you want, installation should be working now.
NOTE: by removing installshield directory, uninstallers for programs which were already installed on your computer will no longer work, and will need to be reinstalled/repaired in order to work.
In any case that you want to recover the ability to uninstall without repairing/reinstalling, recover installshield from recycle bin (which will also bring back the broken uninstall error of Avast and will revive the problem once again. you should recover the directory only if this guide wasn't helpfull for you.