If you see something like the picture below, then your best option is to use the “Completely Remove Virus from the system” button.
You should put all infected files back before running the Virus Cleaner (http://www.avast.com/eng/avast_cleaner.html)
Just after you put the infected files back, you must run the Virus Cleaner.
Once your system is clean, make sure that you let avast! create a Virus Recovery Database (VRDB) to help with subsequent infections.
The files that you mention are system files and windows needs them to run. That may be why winamp does not run anymore.