You can use templates as per below for the Login control. Note that you'll need to provide your own validators etc if you do this, but you're allowed much more freedom :)
If you place a login control in VS, then open up the context box for it, and click 'Convert to Template', you can see what the standard implementation is like.
<asp:Login ... >
<LayoutTemplate>
<asp:TextBox ID="UserName" ... /> ...
<asp:TextBox ID="Password" ... /> ...
<asp:Button ID="LoginButton" ... />
</LayoutTemplate>
</asp:Login>