Win-TV by Hauppague, and several other adapters.. Any Computer store will have them.. They come in basically 3 different types
1: TV-Tuner, analog, takes the RF-out from your DirecTV receiver and decodes it to the TV They all but give away these if they have 'em in stock now days.
2: TV-Tuner, Analog/Digital, Can take Standard RF-NTSC, can take ATSC, or Cable. Generally run 50-100 dollars
Note that cable-digital may be acceptable to type 1 as well in some cases
3: Direct Video Only.. These are designed to accept only video, not RF.
Some may take HDMI (epically type 2) though I've not seen any that do myself (I also have not looked in about 4 years) but the price range is .. Well, just click on the link below.
Best Price USB TV Tuner Video Cards
NOTE: I did not follow the product links to insure they were ATSC compitable, but your DirecTV box should have either a composit (RCA jacks) or NTSC RF out as well.
I do have one of the older USB-TV cards I use for my other laptop (This one can not quite run it)