Hi,
Maybe the sub menu does not support the vertical orientation(check this thread: http://forums.asp.net/t/965523.aspx).
But you can try to use the CSS Control Adapter Toolkit. I have not used it for horizontal sub menus but i believe that is possible.
Or you can use the thrid party, http://www.karamasoft.com/Samples.aspx?component=UltimateSpell
Hope it helps.
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