ComboBox won't work with .net 2.0

<H2>Could not load file or assembly 'System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The system cannot find the file specified.

Stack Trace:

<TABLE width="100%" bgColor=#ffffcc>
<TBODY>
<TR>
<TD><CODE>[FileNotFoundException: Could not load file or assembly 'System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The system cannot find the file specified.]
Newtonsoft.Json.JsonSerializer.Serialize(JsonWrite r jsonWriter, Object value) +0
Coolite.Ext.Web.JSON.Serialize(Object obj, List`1 converters) +286
Coolite.Ext.Web.Store.OnBeforeClientInit(Observabl e sender) +147
Coolite.Ext.Web.Observable.OnBeforeClientInitHandl er() +20
Coolite.Ext.Web.Observable.OnClientInit() +12
Coolite.Ext.Web.WebControl.SetResources() +77
Coolite.Ext.Web.WebControl.OnPreRender(EventArgs e) +371
Coolite.Ext.Web.StoreDataBound.OnPreRender(EventAr gs e) +40
System.Web.UI.Control.PreRenderRecursiveInternal() +86
System.Web.UI.Control.PreRenderRecursiveInternal() +170
System.Web.UI.Control.PreRenderRecursiveInternal() +170
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2041
</PRE></CODE></TD></TR></TBODY></TABLE></H2>
When I try to use the ComboBox control on my page I get the following error... It's strange though, because all of the other ext controls I've used so far have worked. I have .net 2.0 installed... from what I understand from the error message, it looks like a JSON component is looking for .net v3.5? Is there any way I can fix this? Here is what the line of interest looks like in my web.config file, you can see that the correct version, 2.0.0.0 is specified, but the error above is looking for v3.5:

<add assembly="System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>

Thanks!