1203:   file <filename> is not a valid .EXE file

     Note:  This error can occasionally result from the effects of a
     virus.  In this case, first try external overlays (via the SECTION INTO
     xxx command) marking the .EXE read-only, and then follow the procedures
     outlined below.  If nothing else helps, perform an extensive check for
     viruses and take the necessary measures for removal.

     Explanation:  This message indicates that the specified file does
     not have a valid .EXE file header so the Blinker internal overlay cannot
     be located within the file.  This indicates that the file has become
     overwritten by another file, the PATH has changed so that an alternative
     file of the same name is found instead, or the .EXE file has become
     corrupt.