You could write a VB/ADSI script but a the following is simpler:
Export the OU display in AD Users and Computers to a text file
and loop through it with "For /f...in...do..." loop from the command prompt.
Assume OU.txt with first field being machine name
For /f %a in (OU.txt) do @net send %a "Message goes here."
[It's weird, but you don't use a particular idea in an answer for weeks and
then several people ask (unrelated) questions that are easily answered by
the
same idea -- I just gave "for...in...do..." loop answers to two people in a
row.]
