You need an HD TV tuner card:
http://www.newegg.com/Product/Produc...82E16815116015
There's a wide range of types and features. What I linked you is something that will let you use an antenna to pick up the local HD OTA channels, as well as just a cable from a wall directly into your PC... however cable companies these days may require you to use a cable card which would require a different tuner. I'm unsure about how the cable card part works so somebody else can help with that, and it depends what you need.