If you get this error message while trying to install the program, Stuffit, then you have something very wrong.
The install process for Stuffit should not look into any of your folders. It just installs the program and quits.
However, if you are getting the error message when the installer asks if you would like to associate certain file types and if you would like it to search for them, say NO, and move along.
If the error happens when you try to expand the .sit file, rename the file but leave the .sit on it.
If all this fails, attempt to uninstall Stuffit, redownload the program and try again. Make sure you have the correct one for your system. I'm sure you do, but check again anyway

Hope this helps a little