"cannot open for write" midi error on Windows7

[Daisy A]

Hi all,
Hope someone can help me. I have a lilypond file that's working perfectly on a
computer with Windows XP on it but when I drag it to the icon on Windows7 I get
the following error: 

error: cannot open for write: /mypath/myfile.mid: No such file or directory

The file does not exist yet but lilypond seems unable to create it on one
computer when it worked fine on a different one. The only change I made to the
file was to make sure notepad was using utf-8 but I've tested it with other
files which also were fine on the other machine and they don't work either. I've
searched and searched but can't find a solution online, hope someone can help.

If it helps, here is my midi output code: 

  \midi { 
    \context {
      \Score
      tempoWholesPerMinute = #(ly:make-moment 360 8)
    }
  }%-------------end-midi

Thanks in advance, Daisy