You've probably just converted your vs 2005 web project with ajax to vs 2008. This is because Visual Studio 2008 has a newer, built-in version. The fix is short and sweet! 
Remove the System.Web.Extensions from references, then add them back again! That should connect you with the correct assembly. If you're uncertain how to remove the reference, check this out: http://geekswithblogs.net/AskPaula/archive/2008/09/04/124948.aspx