I recently rebuilt my PC combining 2 existing pc’s updating from 32 to 64, changing from win7 to win 8.1 and adding a new large hard disk.
I have now discovered that i have 80 Avast files in assorted locations. What is the best way to tidy this up.
Welcome to the forum. 
Most likely by doing a clean install of Avast which means downloading the needed programs, going off line, then
uninstalling Avast, using the avast removal tool, cleaning up leftovers because of the 2 different OS.
Re-installing the correct version of avast.
See my signature for more details about Successfully installing Avast.
Looks to me you created a mess.
I would wipe all drives and perform a clean installation of windows, all drivers, all applications