Before we can try to help we really need some more information.
What exact version of avast do you have installed/are you trying to install?
What os/service pack is it you are using?
What other security software do you have installed?
How did you uninstall McAcrap and avira?
And please provide the logs as instructed here: http://forum.avast.com/index.php?topic=53253.0

Edit:
Removing the folders/files is not enough. You also need to remove registry entries.