Did you open/create an ArcMap document (*.mxd) on a computer with version 9.2 installed, and now you cannot open the file on a computer that is still running version 9.1?
All is not lost.
In version 9.2, the "File -> Save A Copy As" pulldown menu allows you to select the older software version.
Simply choose "ArcMap 9.0/9.1 Document (*.mxd)" from the "Save As Type" selection options.
Remember to give this copy of your .mxd a different file name to indicate that this is your older version.
Continue your work on this copy in ArcMap 9.1.