ADD CAPTCHA to asp code

We need a solution that will be based on a FREE module of CAPTCHA.
We have several sites, based on a much similar asp code for forms.
1. Regarding the solution – it should fit to
a. adding a module (separate file or directory) without the need to give up any other functionality of the code for exa. The response file who do the "actions" with the data from the form should stay the same
2. the current form we want to add Captcha to:
a. http://g1948.com/clients/FavMapSurveyForm-NEW0711/ContactForm.asp
b.
3. that code of that form is also implemented (asp inside asp.net proj) in :
a. http://seo-sem-israel.com/cms-site/details.aspx?Id=3
4. so need code that will be added to the "form" level but the rest "response" part wont be change..

Of course, the reason is to prevent spam..
Please quote that (please take in care that it mostly putting free module at the right place)