VLC

While I prefer SMPlayer/MPlayer for Windows for most video watching I still have VLC installed on all of my computers. This is because without a doubt it is the easiest player to make a video "play", it may not do it with the prettiest subs, the best performance, etc. but it will almost certainly make it watchable.

All of its codecs are completely self contained, so it will neither cause or suffer from conflicts from other media players or codecs you have installed. It has support for DVDs and even creating a video stream over a network.