Suddenly could not run Enterprise Manager. Located the .MSC file and clicked on it. Still got the same error message:

MMC cannot open the file C:\Program Files\Microsoft SQL
Server\80\Tools\BINN\SQL Server Enterprise Manager.MSC

This may be because the file does not exist, is not an MMC console, or was
created by a leter version of MMC. This may also be because you do not have
sufficient access rights to the file.

Copied the file from another machine. Problem solved.