Javascript Form Question

I am creating a form where clients much first choose a state then a county in that state.

What I would like to do is have two drop-down boxes. The first would have the 50 states. The client must first choose the state, once done it enables the second drop-down boxes which lists the counties in that state.

I’m familiar with how to do this in PHP but I want to be able to do it without refreshing the page, so I figure Javascript is the way to go.

Any suggestions?

Thanks in advance!