Alert message based on record selection

Hi all
My database has an order form which books equipment for meetings. The main

form has (among others), text boxes for Start Date and End Date. The
subform 
has a combo box to select the Equipment. When I select a piece of
equipment 
in the combo box, I would like a msg box or a pop up to alert me if that 
piece of equipment is already booked for that date period. This
information 
is stored in the Orders table. The msg box or pop up should show the
Customer 
Name, Dates, and Invoice number. I would appreciate it if someone will
give 
me an idea of how to do this, or guide me to an example procedure.
Thanks
Ron