Every single video has it's video and audio encoded in a certain way, to play back the video you need to have the correct codec installed on the computer to decode the particular audio and video encoding method, if you've not got the correct codec installed for either part of the video then that part won't play back.
You have two options, either use a player that doesn't rely on the system codecs, such as VLC media player
http://www.videolan.org/vlc/
Or install the codecs that the videos require on both computers, the standard k-lite codec pack should contain all codecs you need for most things (you can untick the media player classic option during setup, it's not required)
http://download.cnet....7171.html?tag=mncol;3