captcha simple code

Hello,

I use Drupal 6.10 and Xampp on windows sp3 for local testing purposes. Started with Drupal a few months ago.
I have seen Rob's video tutorial (btw. it is great) and did so in order to protect my custom html/php mailing form but it does not work. Drupal comment form and others work fine, no matter whether I put image, math or text. Only when I want to use it on my form: nothing happens.

I googled a lot without success, to find some code example because I'm not sure if drupal captcha is supposed to work on custom forms at all?