A possible cause of “‘Sys’ is undefined” explained
Clay Compton explains on the ASP.NET QA blog how bad web.config overrides can lead to the Microsoft Ajax Library not loading and the “‘Sys’ is undefined” error being thrown when the page loads. It’s a great read and hopefully will unblock some of you.
http://weblogs.asp.net/asptest/archive/2008/10/06/asp-net-ajax-and-http-handlers-a-cautionary-tale.aspx