use your original monitor first, then right click on your desktop and go to Properties.  on the Display Properties window, click on Settings.  Set the resolution to a lower resolution first.  try 800x600.  or 640x480.  Apply changes then shutdown (turn off) your computer.  connect your older monitor and power up the system.  check if this fixes your issue.  possible issue is that windows is set to a high resolution when the older monitor cannot support it.