The registry cleaner you mention is the one I've used, first saw recommended on this forum, and recommend to others, to remove a lot of the registry traces of any AV.
(It will also remove at least some of the Avast entries.)
In my experience of using it, it is best to only scan the "H_Key_Local Machine" keys, as a lot of the "current user" keys find stuff that is unrelated to the AV, if any kind of immunity program has been used. (I think.The HKey current user keys shown in the pic appear to be from SpywareBlaster preventing certain domains.)
The registry is the only area it appears to clean, so if there are any active files remaining (in C:\anywhere) I guess they should be nullified without a reg entry. But I don't know that for sure. Things like shared .dll's are a bit of a mystery to me.