Can I switch from .Net 3.5 to .Net 3

After finishing the website that I built in VS 2008 and has AjaxControlToolkit controls. I was informed by the network admin that the server has .Net 3 and that we can't install .Net 3.5.
I tried to change the application settings from Property Page --> Build node, and then select .Net Framework 3.0 from Target Framework Dropdownlist, and then click OK BUT then I can't run my application I get error message and the  
  
 
 
 is unknown even when I try to add reference to the .dll ajax library I still get message inform me that the .dll require later version of .Net Framwrok
 
Any help?
 
Thanks

ScriptManager is part of System.Web.Extensions. If you want to use only 3.0, then you will need a separate download of System.Web.Extensions.dll version 1.0.
If you use 3.5, then it is already part of the framework, and you don't need a separate download.
 
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 hongping:
 then you will need a separate download of System.Web.Extensions.dll version 1.0.

 Would you please provide the link for the file I need to download? Also is there any other steps I need to do after installing this file?
 
Thanks

http://www.asp.net/ajax/ 
You should be able to find more information on that site.
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