How to protect Joomla 2.5 with captcha?

Step 1: Login to admin area

Step 2: Go to Extensions and then Extension Manager.

Step 3: Select Plugin Manager and then Captcha – ReCaptcha.

Step 4: Get the API keys from http://google.com/recaptcha

Step 5: Enter them in the Public and Private Key fields.

Step 6: Save the configuration.