For captcha control you can use this :
To Use this controls just add them in the toolBox By:
Right click on tool Box - >Add tab Now give name to this tab
next Add a control by ->Right click on that tab - > Choose items...
Now locate the captcha dll file and click add to add it to the tab
► To USe it
1. Add line to your .aspx file:
<%@ Register Assembly="MSCaptcha" Namespace="MSCaptcha" TagPrefix="cc1" %>
2. Where needed, add the control itself:
<cc1:CaptchaControl ID="ccJoin" runat="server" CaptchaBackgroundNoise="none" CaptchaLength="5" CaptchaHeight="60"CaptchaWidth="200" CaptchaLineNoise="None" CaptchaMinTimeout="5" CaptchaMaxTimeout="240" />
3. Put a textbox somewhere in your page where your user must enter what he sees in your captcha. Put this code (this example is in C#) to validate user input:
//Inform user that his input was wrong ...
Hope this will help you!