Linksys Wireless Router :: Connecting PC To WRT54G2 V1

I've tried to connect my father's laptop and my father's girlfriend's daughter's PC to my Linksys Wireless Router, model #WRT54G2 V1. So far, I have steady and fast internet access on my PC which so far I've only gotten my father's laptop to connect through. However, I can't figure out how to get her PC to get internet access through my router. I'm mediocre at configuring computers, and I'm absolutely terrible at routers. :/ There was the possibility that her computer isn't new or advanced enough or whatever to be compatible with a wireless router, but at the same time she has a Wireless Network Setup Wizard app. in the control panel area, so wouldn't I be correct in assuming that if a computer has a wizard dedicated to it, it is compatible? I've run Wireless Network Setup Wizard as well as just plain Network Setup Wizard, and I've had them both read that it was successful connecting to my Router's network...but where do I go from there? I've never had to mess with a PC before so I'm not entirely sure. She has a modem, which can not be plugged into the phone jack at the same time as my modem otherwise we both do not have internet access, however if one of us plugs in that person is fine, so that's when I figured I'd try to get her PC into my network with my dad's laptop. Also, we have a Broadband connection setup through Verizon, if that matters at all. Does she need a modem to hook up to my network at all? If so, does that modem need to be plugged into the phone jack? If required I'll give her OS and PC specs and such, but from what I know just being on her computer she runs Windows XP as an OS and her computer is an "eMachine".