Zune only plays WMV (Windows Media Video) format. If you read the manual you will see that they tell you it plays Mp4 formats too but it doesnt, it just has a converter build in to, so when you go to sync it it will take like an hour converting it again to WMV. When you find Find a converter make sure you set it to 240x320 resolution, that messed me up a few times Here's a good list of converters
http://www.zunescene.mobi/forums/index.php?topic=10470.0