The only other suggestion I can offer is to manually run a search for Avast and McAfee and delete anything you find, have you tried running CCleaner to remove any registry leftovers of both AV products.

No need to disable Defender on Win 8.1, it will disable itself when another AV is detected and now that you don’t have a third party AV installed you should at least have Defender active for some protection.

Hopefully you didn’t install any of the Redistributable C++ files to cure the problem that related to “available” Avast no longer needs these installed and is compiled with C++ 2012 which is installed to a specific area, installing C++ manually will not work.

Something else you can try is restoring the system to before you installed avast and then remove McAfee properly first.