Cross-Site Scripting Protection in ASP.NET(A potentially dangerous Request.Form value was detected from the client)

This is basically to  catches malicious scripting code used by hacker.This is simply done in ASP.net by using a filter....
We need to add this in webconfig file as: 

<configuration>
    <system.web>
        <httpModules>
            <add name="ValidateInput" 
               type="Corillian.Web.ValidateInput,ValidateInputASPNET10" />
        </httpModules>
    </system.web>
</configuration>

For more reference use this link
http://msdn.microsoft.com/en-us/library/ms972967.aspx