Enable a WEP password to your wireless router and there should also be some kind of access list where you can type in the MAC Adress'es of the network cards that should be accepted by the router to enter the network.
How to do all of these things should be found in the manual that came with your wireless router...
I can strongly reccomend you buying a Mac with an Airport Extreme router, it's very easy to configure and you'll be up and running in no time with a system that never needs restarting... it'll also work with you're excisting PC's...