I feel so stupid and I need Help!

I just purchased an airport extreme. I have got it setup but for some reason can only use it to connect my Mom's PC, which has no wireless card, directly to ethernet. I can also have it setup for wifi but it only works with my iphone and not my apple tv. I wish I had my macbook but I am waiting for my new pro to arrive so I can't say what it would do with that. Am I missing something in options. Is it possible it only does one thing at a time. I simply can't believe that. Possibly it is something I am missing in setup. To clarify I want to connect my modem directly to the airport express run internet from on of it's ethernet cords directly to my mom's PC, and at the same time provide wifi for my laptop, phone, and apple tv. Any help would be greatly appreciated I have been searching all day and I just want to setup my wirless so i can watch a movie. THanks!