You could add a specific class (or id) to the current menu option, which is what BonRouge's example did (I'm sure you can figure out a way to do this in ASP as well), or add ids to all menu items and use for example a body class to address the accompanying style rules for the hightlighted menu item (do a Google for "body class" to find some additional info on this technique).
The same would more or less apply to sub menus: you could create the entire menu as a two level list, positioning the second level lists absolutely underneath the top level and toggle their visibility in a similar way as you would control highlighting the "current" menu item.
Alternatively, you could have the script leave out the entire sub menus and only include them if they need to show up, but a have a feeling leaving them in may improve findability for search engines (no proof whatsoever for that, though).
By the way: are you having difficulties with generating the required markup, or is it about how to style averything using CSS? Perhaps a more compelling approach would be the sliding doors
one (including "current" tab styles)