I get an error 'A potentially dangerous Request.Form ... ' when I hit submit on my ASP.NET Webform

This error is caused by a feature of .NET Framework, called "Request Validation."
This feature is designed to help prevent script-injection attacks whereby client script code or HTML is unknowingly
submitted to a server, stored, and then presented to other users. To resolve the problem, you can either:

- Disable Request Validation in the web.config file or the ASP.NET page To disable Request Validation for your application,
you must modify or create a Web.config file for your application and set the validateRequest attribute of the section to false
 
- HTML encode the input For more detailed information regarding this feature, please review:

http://www.asp.net/faq/RequestValidation.aspx