asp:XXXXX is not a known element. This can occur if there is a compilation error in the web site.

Problem occurred when I re-install Visual Stdio 2005...after digging for awhile i find the solution. Changing tag prefix did the job.
<webconfig

<pages>

<controls>

<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

</controls>

</pages>

change tagPrefix="asp" to tagPrefix="ajax" or whatever prefix you want. Then change your existing ajax control tags to use that prefix instead. E.g. <asp:UpdatePanel...> becomes <ajax:UpdatePanel...>