Sometimes I get the same problem. It usually happens if I download something that is compressed, then stuffit expander starts working and makes some mess. It decompresses and leaves something unusable on the desktop. In the get info window if usually states that it is a Stuffit Expander document. But I am usually expecting something else after decompression.
I just open that file with ResEdit, and if it looks like an app (which is the case usually if you are downloading an installer file) I end up checking the file type and creator in the get info (under file menu in ResEdit).
I set the file type to APPL, and set the creator to ????. That usually fixes the problem and I can launch the Application, and install whatever I want to.
This only works for Applications, like an installer file.
If you have a document that is compressed and not usable, you cannot set the file type to APPL and use it. In those case I usually set the file type to ????, and creator to ????, and try a couple of different kind of sw hoping that one of them will open it up. For graphics I use Graphic Converter, or Photoshop, for text files I try to use Word. These softwares usually have lot of different translators built in and able to recognize file types.
Hope this helps you and I wasn't too confusing in explaining the process.