Nothing can make the drop-down open when the element gets the focus, except the user. However, you could use javascript and the onFocus event to change the size of the select element so that it displays more then one option.
onFocus
change size
An html select object is NOT the same thing as a VB combo-box, you can not force it to display the list. IE will let you assign a keyboard key that the user can use to display the list, but only possible in IE so why bother.