Setting up the Airport Express on a Desktop PC

Just had to get rid of my airport base station (I still had the one that looked like a UFO, it finally gave up on me). Now I have the Express and I want to use it as my router. So here's my setup:

I have a PC desktop running Windows 7 a MacBook and a Dell PC running XP. I have the Express and a basic modem. I'm confused on where the wires should go in order to let all the computers have internet. I currently have an ethernet cable going from the Express to the modem, but nothing attached to the Desktop (which I don't think has a wireless card). I'm confused because the modem and express only have enough ports to run the ethernet between two things, is there a way I can hook up the modem, Express, AND Desktop??

I'm about to make a genius bar appointment, I need internet asap.