The easiest thing to do is to run Windows Media Centre. The first time it runs it will try to set up the hardware - if it complains that you don't have a TV tuner then that tells you that (a) you don't have the hardware, or (b) you have the hardware but don't have the appropriate drivers for it.
If it complains that you don't have a TV tuner, then run Device Manager (Start -> type "Device Manager" in the search box -> click the "Device Manager" link) and open up the "Sound, video and game controllers" to see what hardware is installed. If you've got a TV tuner in there but no driver then you can right-click it and select "Update Driver Software..."