Configuration Error - install or VISTA problem?

I have just installed the Ajax toolkit and have tried to add an AJAX Update panel to a Content page. I get the below error. Was I suppossed to add something to my WebConfig file? PublicKetTokens were edited for security (was that necessary?)

Configuration Error

Description:An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message:Could not load file or assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=xcxxcxcxcxcxcxcx' or one of its dependencies. The system cannot find the file specified.

Source Error:

Line 20: <add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=fdfddfdfdfddfd"/>Line 21: <add assembly="System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=wewewewewewewewe"/>Line 22: <add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=xcxcxcxcxxxcxcx"/></assemblies>Line 23: </compilation>Line 24: <pages>

Source File:E:\xxx\xxxx_com\web\web.config Line:22

Assembly Load Trace: The following information can be helpful to determine why the assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=xcxcxcxcxcxcxcxc' could not be loaded.

WRN: Assembly binding logging is turned OFF.To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.Note: There is some performance penalty associated with assembly bind failure logging.To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

Just discovered that the below statement was added to my configuration file, I think when I may have added a ScriptManager to one of my MasterPages. When I remove it the website starts working. With it in, the website does not pull up at all. (edited PublicKeyToken)

<

addassembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=vvbvbvbvbvvbvbbvb"/>