@Johnny4745: I booted with a rescue disc (Hirens Boot CD 15.2) and ran command prompt in that guest OS. From there I accessed the HDD and renamed the files. The renaming was just about adding a '_' to the end of each filename suffix.
@ram1220: Since i can't boot the OS installed on the hard drive at all, not even in safe mode, that's unfortunately not an option. I need a way of removing Avast from a guest OS booted from a CD (or USB).