We have absolutely no idea who or what placed the avast DLLs in the system folder.
Anyway, we've changed the avast updater so that starting with the next program update, it will explicitly look for these files in Windows system folders and if found, will nuke them immediately. You're probably not the only ones with the same problem. Maybe some very old version of avast setup/updater had a bug that could cause placing the files to incorrect locations...
Anyway, thanks for helping to track this down,
Vlk