You can enable custom error messages for your ASP .NET application defaulted to Windows/IIS by using a web.config file with a "customErrors" setting configured.

We have included example web.config files below.  For more detailed information refer to http://msdn.microsoft.com/en-us/library/h0hfz6fc.aspx

For Security purposes and coding best practices you should have the web.config customErrors mode="RemoteOnly" or mode="On" instead of mode="Off".  The "Off" mode is primarily useful for debugging, and should not be used in a production environment because of the details it can expose about your server.

To configure a custom error page for 3.5 SP1 and above, follow this example:

<configuration>
   <system.web>
      <customErrors mode="On" redirectMode="ResponseRewrite" defaultRedirect="~/error.aspx" />

   </system.web>
</configuration>

This example is for ASP.NET 2.0 - 3.5 without SP1:

<configuration>
       <system.web>
          <customErrors mode="On" defaultRedirect="~/error.html" />

       </system.web>
    </configuration>

ASP.NET Security references:

• Important: ASP.NET Security Vulnerability:
  http://weblogs.asp.net/scottgu/archive/2010/09/18/important-asp-net-security-vulnerability.aspx
• Understanding the ASP.NET Vulnerability:
  http://blogs.technet.com/b/srd/archive/2010/09/17/understanding-the-asp-net-vulnerability.aspx