Xp media centre is very good, stable and works full stop as Turin says.
It does get a little more complicated from there on the windows platform. 7 media centre is also great and very good looking & the interface is designed specifically with widescreen tv's in mind in that the various menus scroll horizontally whereas in xp they run vertically - seems a small point but in use it makes a difference.
With Vista, the media centre app is slightly more flaky (as you might expect) than in xp or 7, but and it's a fairly big but, there is an add on called tv pack which adds a lot of functionality to the thing, most notably automatic subtitling on demand for live tv, so that if you mute the sound, subtitling begins and interactive services which in the uk means the red button again on live tv. Conditional access modules are also given support in tv pack.
So if you're thinking of going with Vista, I would definitely seek out tv pack as an add on, but sense suggests avoiding Vista altogether and going with 7 if windows is the route you're going down.