Windows 7 (most versions) have Windows Media Center, which does have TV functionality. And of course, you will need drivers for your TV tuner (PCI board, USB)
If you need the software for your existing TV tuner you can download it from the manufacturers website. If your looking to download a software TV tuner, there is no such thing. TV tuners are hardware devices.
If you have a Pinnacle tuner you can download the software from PCTV since Pinnacle sold off their hardware division after it was purchased by Avid.