Akismet plugin comes by default with any new WordPress version. As you probably already know, WordPress blogs are heavily spammed. I really think that you should use Akismet instead of boring and non-accessible captchas to prevent blog spam.
As I said, Akismet comes bundled with every WordPress release and so you don't have to download it. The first step is to activate it. Login to your WordPress dashboard and click on Plugins. Scroll down until you see Akismet in the plugin list and click on the Activate button to activate it.
Take note that Akismet is now activated, but it isn't running yet. You have to provide an API key. The API key is free, but you have to create an account at wordpress.com in order to get one.
Go to www.wordpress.com and create an account.
Login to your wordpress.com dashboard and select Edit Profile on the drop-down list that you get when you hover the mouse on My Account.
Your API key is listed here...