internet connection problems sharing

Got a new Mac Book for Christmas, and I am still struggling with the differences compared to my PCs. Biggest problem so far is difficulty sharing sbcglobal DSL internet connection. The plan was to use new Airport Extreme to connect Mac Book viw wireless, but still use hardwire connection through Airport Extreme so we could still use use old PC and share internet connection. I hooked the DSL modem cable into the airport extreme (after configuring it with the Airport utility program). After restarting DSL modem, Airport Extreme shows solid green light. However, after going through the process of setting up network (I guessed at some of the answers at the prompts) I got it close enough that it apparently connected to the internet just long enough for an AT&T window to open up telling me I needed to download software, and then it won't connect again to actually download it. Everytime I get through the diagnostic program on the Mac it tells me it can't solve the problem as the computer is simply not connected. Second problem, when I hook a network cable back from the Airport Extreme to my old PC through the port on the back, it won't connect to the internet either. Any ideas?