Sleeping is necessary [Frown] and requires more than 0.5 seconds or even 0.75. Sleeping for a second seemed to do the job.

code:
 
function RefreshDesktop()
  dim $shell 
  $shell=createobject("shell.application") 
  $shell.MinimizeAll
  sleep 1
  $=sendkeys("{f5}")
  $shell.UndoMinimizeAll
endfunction
_________________________
(... better days ahead)