All the programs in Add/Remove Programs have a corresponding registry entry. If the program's installed but it's not showing in the list, the Avast registry entry may have been deleted for some reason. Some registry cleaners will do that. I agree with Coolmario, go ahead and boot to safe mode and run aswClear. Before you install the new version, you might check in C:\Program Files\AVAST Software\Avast and see how many files and folders are there. If it uninstalled correctly there should only be a few (if any). I've got 50 files and 6 folders with Avast installed.