Hi Shama,
There is an excellent Captcha plugin for Joomla that can be used with the Joomla registration form. It's called reCaptcha.
It's access URL is:
www.bestofjoomla.com/component/option,co...nk_id,685/Itemid,95/
There are multiple Captcha plugins for Joomla but my favorite is re-Captcha. It's effective, well documented and simple to install and configure.
If you have any problems with re-Captcha do post to the forum, I'm sure that there will always be someone to help you out.