How to add a custom antispam questions in User registration Form
You can also customize captcha by the CAPTCHA Pack module. Here is a quote about it (from the module's project page):


  ... contains several CAPTCHA types for use with the CAPTCHA module. The CAPTCHA Pack module is meant to provide lightweight, yet effective alternatives for the traditional image CAPTCHA, which is undesirable in certain situation (e.g. bandwidth restrictions, cpu restrictions, accessibility constraints, etc).