Microsoft Windows XP Media Center Edition 2005 OEM

We used Microsoft Windows XP Media Center Edition 2005 (MCE 2005) for our Media Center PC and are very happy with the ease of use and reliability, both lacking in a competitors (now discontinued) product. The optional remote control works well as does the optional wireless keyboard to a lesser extent.
This older Operating System is still a good candidate for a Media Center PC as it uses a Media Center application that is very similar to that found in Microsoft Vista Home Premium and Ultimate editions without the larger overhead and performance penalty that Vista can exhibit.   In particular, because XP is less demanding than Vista, one can build a Media Center system that has a less powerful CPU that generates less heat, which means less distracting fan noise and lower power consumption. 
Of course this XP-based Operating System has all of the XP weaknesses, vulnerabilities and limitations. One of which is that it will never officially support Blue Ray playback.
If what you want is a great Media Center experience on a PC, Microsoft has really great Media Center software both here with MCE 2005 and with the newer Vista H.P. and Ultimate editions. Having used both, I'd say that MCE 2005 is the better choice for most current mid-level systems and Vista 64bit could be a better choice for extremely powerful hardware and for eventual Blue Ray compatibility.