I should also mention there's a decision to make between using "element.style.visibility = 'hidden';" to hide the drop-down box, or using "element.style.display = 'none';".
Each is suited for a diferent purpose. visibility='hidden' will mean the element still takes up the same space it would otherwise take up, it's just invisible. display='none' means it looks as if the element were never there.
So it depends what effect you're going for. You can see an example of the first method in Fang's post, and my example uses the second method.
Hope this helps.