A couple of ideas . . .
Did you use the PrevInstance in the Form_Load event of the first initial form loaded? This appears to work on my machine as you've written it in this event.
Otherwise, you could use a Win32 Mutex resource. Get exclusive use of the first instance, and then deny access to the resource for all subsequent instances. You'd have to be careful not to leave all of those instances queued up though.
Hope this helps
