ajax rounded corners

Can anyone help me with implementing an ajax rounded corners control on the header (or anything else really). I've never used it before. When I copy it onto a page it asks if I want to over write the current AjaxControlToolkit.dll and if I do the whole system fails. It seem to because it is a new version, but I've very new to the ajax thing. I also dont know how to connect it to the header. Whether I use one of these ID's
            
<nopCommerce:Header ID="ctrlHeader" runat="server" />
<nopCommerce:HeaderMenu ID="ctrlHeaderMenu" runat="server" />    
<asp:ContentPlaceHolder ID="cph1" runat="server">

in the master page to or if I connect it to the header module some how.
Any help or suggestion would be great.

I can't remember if they reference the Toolkit from the web.config or not, but I believe they do.

1. If you overwrite the current toolkit with a new version dll, you'll need to update the web.config with the new assembly PublicKey or remove it entirely and make sure you are just referencing the Assembly name.  Check this post:
http://forums.asp.net/t/1174924.aspx

2. The rounded corners control basically just adds a top and bottom to a Panel or Div.  In my testing of this control it only worked on Panels/Divs.

You could also consider using jquery round corners.  http://www.methvin.com/jquery/jq-corner-demo.html