If you have problems watching videos in Windows, this should be the ultimate solution:
Download and install the K-Lite Codec Pack:
http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm
Make sure that during the installation the H.264 support of ffdshow is turned on (I think that in at least some versions of the pack it was turned off by default). During the installation it will offer you to uninstall unneeded codecs/players already installed in your system (to keep it clean); afaik it's safe and a good idea to let it do that.
If you still have problems viewing the videos, play them using media player classic or bsplayer, both of which come with the pack. Winamp5 also is able to play most videos (except multi-soundtrack ones). Sometimes just having the codecs installed is not enough for a player (such as Windows Media Player) to be able to correctly play a video; media player classic and bsplayer should support basically everything.