And here is the uninstall link http://files.avast.com/files/eng/aswclear.exe which is to be run in safe mode after uninstalling avast normally if what DavidR has suggested doesn’t work and then you might have to do a clean install.
Uninstallers for other av’s here if you never ran one after deleting your old av http://thewebatom.net/uninstallers/security-software/