Try looking at the settings in VLC,when you install VLC you have the option to make it your default player for a whole range of file types (Similar on widows media player but not so many choices)
Also if you use 'Search' on forums and type in VLC you will see that it comes highly recommended by a lot of people, and it plays more file types than many other players, and lastly (Which I like) it uses far less CPU than other players. hope this helps
N2