I want to protect a part of a website from bots and other automated "crawlers" with a form of human character recognition (sic).

The page can be either php or, I think, Asp on Linux. I'm assuming this has been done tons of times before and considering that I know s h i t about php, a semi-turnkey solution would be preferable

To (attempt to) clarify what I'm looking for I'll give an example:
-> The page displays a randomized prompt such as [1] [7] [0] using 3 bitmapped cells
-> The user user types "170" into an html <input ..> field and hits a submit buttom to proceed to the destination page. If the user gets it wrong they are returned to a new randomized prompt where they can try again to type in the new number <rinse & repeat until they get it right>.

I've seen this on various signup pages for anonymous web-based email accounts and the suchlike before (not that I can readily find an example to help explain my request here, typical!.

Can anyone point me in the right direction please?