ISDone.dll - An error occurred while unpacking! Unarc.dll returned an error code: -6

This error may be caused by a missing version of Visual C++ on your computer. Therefore, install the common redistributables.

Also, move the setup folder to the root of your disk, and select to install the game on the root of your disk as well:

After that, start the installation again.

If it is a FitGirl game, also activate the RAM memory limiter at the beginning of the installation setup:

If the error persists, install the game from another source that makes its games available in a pre-installed or portable format.