Microsoft’s AjaxToolKit is a very handy thing to have for any ASP.NET developer.  However, one of my biggest problems with it is that I have encountered problems in IE6.  The position of the modal appears way off center.  After doing a little research, I’ve found the best way to solve this is to wrap the panel that you’re using for the modal in a div with a style of “position:relative;_z-index:1” (it’s important to note the underscore in front of z-index. This targets IE6 specifically).  You can also give the div a class and set the z-index in a separate IE6 stylesheet if you want to still keep your CSS valid.  Here is an example of this fix in action:

<asp:HyperLink ID="lnkModalTrigger" runat="server" NavigateUrl="#" Text="Click Me!" />

<div style="position:relative;_z-index:1;">
<asp:Panel ID="pnlModalContent" runat="server">
     <asp:HyperLink ID="lnkModalClose" runat="server" NavigateUrl="#" Text="Close" />
     <p>Here is some text that will appear inside the modal window</p>
<ajaxToolKit:ModalPopupExtender ID="Modal" runat="server" PopupControlID="pnlModalContent" TargetControlID="lnkModalTrigger" CancelControlID="lnkModalClose" BackgroundCssClass="act-modal-blackout" />

This should fix any positioning problems that you may be having in IE6.