You can use templates as described below for the Login control. Please note that you will need to provide your own validators, etc. If you do this, but you are allowed much more freedom :)
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<asp:Login ... >
<LayoutTemplate>
<asp:TextBox ID="UserName" ... /> ...
<asp:TextBox ID="Password" ... /> ...
<asp:Button ID="LoginButton" ... />
</LayoutTemplate>
</asp:Login>