Windows XP Video Decoder Checkup Utility
UPDATE: As of January 2009, the easiest way to play MPEG-2 files in Windows Media Player is to follow these steps. You can still use the Windows XP Video Decoder Checkup utility to find issues, if you prefer that approach.
If you have trouble playing MPEG-2 files (DVDs, SVCDs, MS-DVR files recorded by an XP MCE machine) in Windows Media Player 10, this app may help you identify compatibility issues with the MPEG-2 decoders on your system. According to Microsoft: "If you encounter a problem while using Windows Media Player 10 to play a DVD or to synchronize (copy) recorded TV shows to a Portable Media Center or other device, use this utility to verify that you have a compatible MPEG-2 decoder installed on your computer." What I found was that out of 15 codecs I tested, none of them were completely compatible with Windows Media Player 10. Before you get alarmed, I should point out I have no problem watching movies from DVD or MPEG-2 files with the current codec set on my system. The machine I did the majority of my testing on does have occasional issues converting MPEG-2 video to WMV format using Windows Media Encoder, but that was true prior to the installation of Windows Media Player 10.
Tracking down a valid codec proved a little trickier. The utility suggested a newer version of an NVIDIA codec on one system might be compatible, but I could not locate a newer version on NVIDIA's Web site. My InterVideo codec made a similar claim, so as a test, I purchased the Microsoft approved (according to InterVideo) DVD decoder from InterVideo. That option proved to be a waste of $20. InterVideo tech support suggested I spend $50 on a newer version of WinDVD, but when the $20 problem didn't fix it I find it hard to justify throwing more money at the problem. Several of my MPEG-2 codecs are from Sonic, thanks to MyDVD and DVDit. None of the Sonic codecs are compatible with Windows Media Player 10, which Microsoft thankfully warns on the recommended DVD decoder page. In an effort to be complete in my search for a Windows Media Player 10 compatible MPEG-2 codec, I purchased the Cyberlink codec listed on the DVD decoder page at Microsoft expecting similar results to my InterVideo experience. Fortunately Cyberlink actually passed the test. Bottom line: This is a nice tool for identifying codecs on your system. If you are experiencing any weirdness playing MPEG-2 video with Windows Media Player 10, take a look at the Cyberlink codec, otherwise you might want to use a player more suited to the codecs on your system.
As an aside, I'd love to hear about codecs on your system that do pass the compatibility test. Please click the feedback link in the sidebar and let me know the company, codec name, and codec version number if you find one.