Dropdown list...easy one..

Question

Hey folks, All I want to do is restrict a dropdown list such that when a user hits the OK button on a form it will verify that.... The dropdown list on the form is not null the dropdown list does NOT contain text that was just typed in by the user I can easily do a check against character length in to text property of the dropdown to ensure it is not blank, null etc. However, how can I prevent people from just typing something in? How can you set the combo box to default to one of the items within its collection? Thanks, Bob