"CAPTCHA is a type of challenge-response test used in computing to determine that the user is not run by a computer." Many services on the web use a CAPTCHA test to determine if the a user is legitimate. A common test is to show a distorted image and then the user will respond by typing the text of the distorted letters.
The security labs at websense.com did some interesting work how the CAPTCHA test for Live Mail (a Microsoft e-mail service) is being subverted (link: http://securitylabs.websense.com/content/Blogs/3063.aspx). The article shows how a "victim" computer is used to create the accounts. This is all the more reason that users should be wary and take security of their own computer seriously.