Based upon your description, I'm assuming that you're trying to achieve the drop down effect via the :hover pseudo class. IE6 does not supprt pseudo classes (like :hover) on anything but the anchor <a> tag.
Okay, I've played around with it, and now the menu items (apear) and (b) line up nicely in IE 6 and 7, but they are pushed over to the left side of their container on FF and Opera 8.5.