There are two ways of doing this:
1) buy a TV tuner card. Plug it into your PC, hook it up to your cable TV box, and you should be able to watch TV on your computer.
2) if your monitor has multiple inputs and your cable box has DVI-out, you can connect the cable box directly to your monitor, and you can switch the monitor back and forth between displaying TV signals from the cable box or displaying your PC's screen.
Hope this helps.