He'll want to get a TV tuner card from Hauppauge for his Windows computer. He'll then plug in the cable from his antenna (or cable or satellite, if he had that), into that TV tuner card. Then he can use Windows Media Center to watch TV, or Hauppauge has software he can use to record and watch TV.
For the Mac, there's software called eyeTV from Elgato.
If Matt wants to set up a home theater PC, he should watch this episode of Know How.