See Igor’s answer. If you can work with your system (boot, run applications…) so it’s safe to delete that files. If you’re not sure, let them there for a while. They are safe there and cannot infect your system.

If you said so… the infection was spreding into your system.
How are you right now? Did you run a full scan?

Some files were posted there by avast (backup purposes) other could be infected.
Like Igor said, we need to know the names (and path) of that system files.

See User’s FAQ link on my signature and, please, browse there a little until you find VRDB explanations. Anyway, it’s an automated way, you can’t do (manually) so much 8)