I tried to uninstall using normal Add/Remove Program, got error The stub cannot run installer/updater executable ‘C:\Program Files\Alwil Software\Avast5\setup\Sfx\avast.setup’(code 0x3)

I then tried using Avastclear, still got the same problem/error i.e The stub cannot run installer/updater executable ‘C:\Program Files\Alwil Software\Avast5\setup\Sfx\avast.setup’(code 0x3)

Any ideas?

Try the old uninstaller (AswClear.exe) provided by DJBone.
https://forum.avast.com/index.php?topic=60523.msg552606#msg552606