Apple Hardware Test will not start up

I am using a mid 2007, 20" iMac running the most recent software and all my updates are installed. I think I have a video issue and wanted to run apple hardware test, but it won't start up. I have restarted more than a dozen times pressing the "D" key each time, but the test will not start... it just runs through a normal startup leading to the login screen. Any thoughts as to why I can't run it or what I can do to get it going.