If you're running Windows 7 Home Premium or Windows XP MCE edition, then you already have the necessary software installed to view cable content. You do, however, still need the hardware.
Pretty much any TV Tuner card that has at least one Clear-QAM tuner will work for your needs. I would also recommend tuner cards from Hauppauge as well as those from AverMedia. Which ever one you get, I'd recommend getting one with an MCE Remote Control.
Edit: @AM2A - True that much of basic cable is available online through applications like Hulu (I don't think Netflix is all that current in their content), but I prefer a TV Tuner to watch live TV and there's plenty of content that simply isn't available elsewhere (live news, sports).