Failed to load class from assembly "system. Web. Extensions, version = 3.5.0.0, culture = neutral, publickeytoken = 31bf3856ad364e35"

The Asp.net Ajax program developed in the notebook is released to the Windows 2003 Server with an exception:

[Typeloadexception: failed to load type "system. Web. Extensions, version = 3.5.0.0, culture = neutral, publickeytoken = 31bf3856ad364e35" from the assembly "system. Web. UI. scriptreferencebase".]
Ajaxcontroltoolkit. toolkitscriptmanager. onresolvescriptreference (scriptreferenceeventargs E) in D:/References/AjaxControlToolkit-Framework3.5/ajaxcontroltoolkit/toolkitscriptmanager. CS: 189
System. Web. UI. scriptmanager. registerscripts () + 261
System. Web. UI. scriptmanager. onpageprerendercomplete (Object sender, eventargs e) + 117
System. Web. UI. Page. onprerendercomplete (eventargs e) + 2063008
System. Web. UI. Page. processrequestmain (Boolean includestagesbeforeasyncpoint, Boolean includestagesafterasyncpoint) + 2247

The possible cause is that the. NET Framework 3.5 SP1 beta is installed in the notebook, but not on the server.
The ajaxcontroltoolkit is used in this program. It is estimated that the relevant part has a problem.

Http://forums.asp.net/p/1273185/2412702.aspx