This could be one of two things…If all of your file types are opening up one program but your MP4s are opening up a different program (quite possibly Quicktime), then it may be a video overlay problem — try upgrading your DirectX installation (www.microsoft.com/directx for more information), or try turning your graphics acceleration on or off (depending on its current setting). It could theoretically also be wonky codecs, especially if MP4s work okay — if you’re having the problem on the same type of file over and over again (always Windows Media for example), you’ll need to upgrade your codec for that particular file type.