Hi,
There are three main approaches to dropdown menus
The first method and the most commonly recommended approach is to use a suckerfish dropdown, this involves formatting a list using CSS into a dropdown, however IE5-IE6 can't display pure CSS dropdowns correctly, so a little javascript is necessary to kick the menu down in these browsers. Here are some links to good tutorials on Suckerfish's:
http://www.alistapart.com/articles/dropdowns/
http://www.htmldog.com/articles/suckerfish/dropdowns/
Another approach is to use nested tables inside IE conditional comments to kick down the menu in IE5-IE6. This has the advantage of not requiring javascript for these older browsers, however it requires more code and your HTML can get quite messy. There are some good examples available at CSS Play:
http://cssplay.co.uk/
Another approach which is definitely not recommended, is to use javascript for ALL browsers. This is an older technique and is not used anymore.