The very idea of turning your PC into a TV might seem a little nutty. After all, what family wants to gather 'round a smallish LCD in the den to watch Chuck or The Office? And what living-room entertainment center would look good with a desktop computer wired to the HDTV? Fair points, to be sure, but consider this: A PC equipped with a TV tuner can also record shows, TiVo-style, and then burn them to DVDs for archiving. Some media centers can also copy recorded shows to such portable players as iPods and Creative Zens, which TiVo charges extra for.
If your PC happens to be a notebook, you can watch recorded shows while you're on the road (and mock fellow travelers who paid $1.99 on iTunes for a single episode of Battlestar Galactica). Finally, you always have the option of connecting a set-top "extender" to your living-room TV, giving you all the goodness of your media center without your actually having the computer there (more on that later).
The key ingredient is a tuner. Relatively inexpensive and easy to install, they're available in both PCI (internal) and USB (external) flavors. They support both analog and digital sources, too. An analog tuner can receive standard-definition cable or satellite signals, while a digital tuner affords access to over-the-air (OTA) digital broadcasts and/or QAM signals; the latter are unscrambled digital channels (usually just local ones, but high-definition) delivered via standard cable.
The OTA option is good because it provides high-def channels free of charge (bite me, cable company!), though it requires a decent antenna. (Visit AntennaWeb.org to see whether a simple set of rabbit ears will suffice or if you need higher-end hardware.) If you're already subscribing to cable, QAM may deliver the same high-def channels, no antenna required. But it might also require you to use the tuner software that came with your tuner card to view and/or record broadcasts, as not all media-center programs natively support QAM.
The Hauppauge WinTV-HVR-2250 is a PCI Express card that comes with a pair of hybrid digital tuners, meaning they can decode both analog and digital signals (QAM included). In addition, it comes with a plug-in for watching and recording QAM channels via the Windows Media Center program guide--a major plus for Vista users.
If you need an external tuner, consider the SiliconDust HDHomeRun, which provides a pair of digital tuners for either OTA or QAM broadcasts. Interestingly, the tuner connects to your home-network router, not to your PC, so it can pipe live TV to nearly any media-center PC in the house. Plus, it's compatible with both Windows and Mac systems (though the latter require Elgato's EyeTV, sold separately).
Want something a little more travel-friendly? AVerMedia's AVerTV Hybrid Volar Max is a USB tuner about the size of a flash drive,