How can I perform an Apple Hardware Test without my original install media? I have a 2007 MacBook that I'd like to perform an Apple Hardware Test on, but I don't have the original install media. I have a Snow Leopard install DVD, but rebooting and holding down D does not launch the test. Short of going to the Apple Store, is there any way to test the hardware?
I've never been able to figure out why this function seems so unreliable - holding D for diagnostics doesn't seem to work for me in practice. I wish I had a better explanation than "many models don't have this and even the ones that do, this isn't something I can count on."

I have come to rely on AHT that are available for authorized Apple technicians to download so you might be able to get help from one closer than an Apple Store - http://www.apple.com/buy/locator/service/

Apple will usually run a very good test for free as past of a genius bar appointment, and both Apple and Authorized Service will usually check a mac in for diagnostics under coverage for no charge. Out of warranty, expect to pay $39 to $85 for a diagnostic.

Lastly, you can order replacement media if yours are misplaced from Apple. 800-APL-CARE