QuickTime Pro

Windows users should be able to play back an MP4 (they normally contain both video and audio). Otherwise, you could try the free VLC Player and try converting it to a different format. At least, I think you can use it for conversions. I normally only use it to play back video files.

Apple stopped selling Pro licenses for QuickTime 7 about two years ago. The current version of QuickTime installed with the OS has a few features the old Pro version didn't, but still lacks quite a few of 7's features.

Yes, if they download the Windows version of VLC, they should be able to play the file as is.