CAPTCHA stands for Completely Automated Public Turing Test To Tell Computers and Humans Apart is used to ward off automated bots/spammers from repeatedly doing the same task.
CAPTCHA is felt as a secure mechanism to prevent comment spam in Blogs, protecting repeated website registration, prevents dictionary attacks. After reading this blog I realized that captcha cannot be trusted for security. It seems they are pretty much easy to exploit. It appears that now spammers have found a way to overcome CAPTCHA systems, and they have created 15,000 bogus Hotmail & Yahoo accounts. So if you are using captcha to exploit automated spammers, reconsider your approach and follow some other security measures. One good method for dealing with the problem would be to use Bayesian filtering which is considered as the most effective Spam filter. If u would like to learn more about Bayesian and other Spam filtering techniques do check out this article(s) by PaulGraham.