'Sys' is undefined trying to run an AJAX UpdatePanel

When you try to run an AJAX UpdatePanel and are receiving the Javascript error 'Sys' is undefined, make sure you have checked these conditions:

1. Your Web.config file is extended for the use of .Net 3.5
   (Visual Web Developer 2008 Express Edition will ask you to update your application when opening this the first time).
2. You have an application extension .axd pointing to C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll with the option Verify that file exists unchecked.
3. There is no wildcard application map configured on your application.