Unarc.dll or Isdone.dll errors are quite common for both Windows 7 and Windows 8 (8.1) users. The fact is, they are almost one and the same error. These errors can occur when you, for example, download an archive from the Internet and try to unpack it, or try to install a downloaded game with the Innosetup installation program.
If you search for a fix on the Internet, you’ll find many “unarc.dll fixer” programs, but none of them work. I highly recommend that you not download any of these applications. Working for quite a while as an IT support engineer, I can give you some helpful advice on how to fix the unarc.dll and Isdone.dll errors.