How to use one field in a form to affect another???

Hi all

I'm fairly new to seriff and web design so forgive me if this has been gone over already (although i have trawled through google for answers).

I have created a form for my website and I need users to select an item within a combo box, and for that selection to disable other items within the form ie, other combo boxes.

Does anyone have a solution to this?

Thankyou in advance for any help)