If you install a Maxis game called SimTown, you may notice the “Show Desktop” icon turned into a SimTown Sound File. However, going into the file extension manager will not work. Here’s how to fix it.
Go into the registry editor (Start >> Run >> regedit). Then go to HKEY_CLASSES_ROOT\SHCmdFile\shell\open\command. SCF files should be associated with explorer.exe.
Next, go into the command Prompt, which is usually under Start > Programs > Accessories. (‘Start > Run’ won’t work for this!) Type in “assoc .scf=SHCmdFile” and it should restore it.