AJAX: Element 'CollapsiblePanelExtender' is not a known element.

I get this error no matter what AJAX control I attempt to use. As long as I'm in design mode in Studio 05, I get no errors, but the second I go into source mode, there are the errors, on every AJAX control. And there is no Intellisense on any of the controls.

 My code:

<

asp:ImageButton ID="btnTools" runat="server" ImageUrl="~/images/menuTools.gif" BorderStyle="None" />
<asp:Panel ID="pnlTools" runat="server" Width="204px" CssClass="clsNinetyFivePercentFont">
    <asp:HyperLink ID="hlT01" runat="server" NavigateUrl="#" Text="<span style='text-decoration: none !important;'>-&nbsp;</span>Checklists/Team" CssClass="lnkWhiteToPinkBold clsNinetyPercentFont clsUnderline" /><br />
    <asp:HyperLink ID="hlT02" runat="server" NavigateUrl="#" Text="<span style='text-decoration: none !important;'>-&nbsp;</span>Close Milestone Status/News" CssClass="lnkWhiteToPinkBold clsNinetyPercentFont clsUnderline" /><br />
    <asp:HyperLink ID="hlT03" runat="server" NavigateUrl="#" Text="<span style='text-decoration: none !important;'>-&nbsp;</span>SOX Management" CssClass="lnkWhiteToPinkBold clsNinetyPercentFont clsUnderline" />
</asp:Panel>
<cc1:CollapsiblePanelExtender id="CollapsiblePanelExtender1" runat="server" ExpandedSize="150" TargetControlID="pnlTools" CollapseControlID="btnTools" Collapsed="True" CollapsedSize="0" ExpandControlID="btnTools" SuppressPostBack="True" />
The control works, but won't compile. I just have to save and view the site in the browser.

I'm using these collapsible panels to replace a treeview menu for my site. I've noted another bizarre error: if I remove the treeview completely from the file, I get a massive .Net compilation error when I try to view the page. But if I include an empty treeview with a false visibility (<asp:TreeView ID="treeMainMenu" runat="Server" Visible="false" />) right before the ImageButton shown above, then all is well, and the page will render.

 I did a fresh install of the Essential Components and the Control Toolkit today (first install on this box).
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