Conditional contents in a ComboBox

Gotta love the alliteration.
Hi all, first post and all that, so please be gentle.

I’m trying to learn AS3 the hard way (i.e. just barrelling on into it), and I’m a bit stuck on this form that I’m putting together. Aside from the normal headaches associated with form building in Flash, I’ve got a series of conditional contents that I need to work out.

Basically, it’s a no-brains-required error reporting form, and it’s covering three sites, with several buildings, and a couple of thousand computers.
What I want to do is to limit the amount of selectable machines available as more fields are filled out in this form: say I chose site 1, I only want machines that are actually on site one to be available. Would it make more sense for this to be done at the server end, or is there a way of getting the combo boxes to check to see whether conditions have been met prior to displaying contents?

The only option I’ve gotten with them so far is a fixed list of entries.
Thanks in advance.