Download a freeware app called Media Player Classic.
http://www.afterdawn.com/software/vi...er_classic.cfm
It's a no frills media player that plays pretty much everything.
It's also very light and doesn't bombard you with advertisement crap or 'editors picks'.
If you install the Quicktime alternative codecs it can also play quicktime...however it won't support the PRO functions of quicktime so if you have Quicktime Pro be careful with installing these codecs.
Good luck,
Joe