Are you using Windows Media player 11?
Whether you have or not yet, there are some positive upgrades such as the sleek new look, revamped playback controls, sync with your mp3 player and more intuitive media management for your large collections of video, audio and even album art.

However, like with anything, take the good with bad, and for Windows Media 11 there is no exception. After trying it myself, the first thing I went to test was video codec support. I found it to be lacking. This has been a big issue in previous versions of Windows Media players. Audio files are no problem, images, not an issue, its the video type support that falls short. Again, unless you are playing a standard AVI or WMV file for example ,there are issues with other codecs such as Xvid, DivX, Mov and so on. Many have reported green lines on their AVI video due to problems with the native vista codecs. So whats the solution if you want to keep using windows Media player 11 you ask? An Advanced Vista Codec Pack v4.4.8 has just been released containing all the codecs you would possibly need to play your new and old video files. Its a free download. Get it at Softpedia.com , there are instructions there