Tuners come in two basic varieties: internal, which plug into PCI slots, and external, which rely on USB. Ideally, you should choose a tuner that has both NTSC (analog) and ATSC (digital) inputs. The analog input is where you'll connect your cable or satellite box, while ATSC is for an antenna.

The Alienware Hangar18 media center PC looks like a stereo component, so it should be right at home in your living room.
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Yes, an antenna: ATSC tuners let you receive over-the-air, high-definition broadcasts from any TV station in range. These are free signals; you don't have to pay the cable company a dime. Where you live will dictate the kind of antenna you need; visit AntennaWeb.org to see if you can get by with a basic set of rabbit ears or whether you need outdoor hardware. Either way, it'll still cost you considerably less than cable or satellite HD.

What if you already get HD programming via your cable or satellite box? Can you integrate it with a media center? Short answer: not yet. While Vista Media Center works fine with over-the-air HD, it can't capture HD content delivered through a converter box. The solution lies in a new technology called CableCard, which replaces the box altogether and enables Vista to tune in and record both regular and premium HD channels. Unfortunately, only one CableCard tuner has been announced so far -- ATI's TV Wonder Digital Cable Tuner -- and it's been delayed for more than a year. So you'll have to wait to turn your media center into the ultimate HD DVR.

In the meantime, there are plenty of NTSC/ATSC tuners to choose from. The $130 Hauppage WinTV-HVR-1600, for instance, is an internal card that offers a pair of tuners (one analog, one digital). For an even more versatile solution, check out the $169 HDHomeRun, a dual-tuner box that connects to your network and then streams TV programs (standard definition and HD alike) to one or more PCs.

Windows Media Center's program guide is provided free of charge; you don't have to pay monthly fees like with TiVo.
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It's a fairly simply matter to connect your media center to your TV. (If you haven't yet purchased an HDTV, check out InformationWeek's HDTV Buyer's Guide 2008, which will help you understand the costs, options, and terminology.) Assuming your video card has an HDMI port as described above, just connect an HDMI cable and plug it into a corresponding port on your TV. If not, it probably has a DVI port, in which case a DVI-to-HDMI cable will do the trick.

Of course, this assumes you even want a PC connected to your TV. Unless you chose a model designed like a stereo component (the Alienware Hangar18 is a great example), few PCs fit in with living-room decor (or fit into entertainment centers, for that matter). To keep component clutter to a minimum, you may want to consider a media center extender -- a component-style device that streams video and audio from your PC (which can be installed in a less-conspicuous spot) to your TV.