How do you open MP4 files on Windows? I was recently sent some video clips in .mp4 file format and could not open them with Windows Media Player, which displayed a message saying the file was incomplete or corrupt. I tried various remedies as suggested by Windows Help but to no avail. Eventually I downloaded a program called VLC Media Player and the files opened with no trouble. Should I have received some updates from Microsoft to overcome this problem? Windows Media Player doesn't natively support the .mp4 format, so you haven't missed any updates. If you want an easy life I suggest that you stick with VLC, which is very versatile and copes well with most popular media formats but it is possible to get WMP to play .mp4 files. You need to download and install a third-party codec pack, but be warned that it is an unapproved procedure, and you do so at your own risk. The two packs that work best are the Combined Community Codec Pack (CCCP), which is free from http://goo.gl/15DBY, and K-Lite, which is also free, from http://goo.gl/Meknl.