Don't go wild installing all sorts of silly codecs you don't need.
Open each file in Gspot (google it, free) and find out what codecs you NEED.
Install what you need only.
No, they're not like video card drivers.
Just make sure you only have ONE version of each codec you need.
Two versions of Xvid can royally mess up your video playbacks.
Get the codec's here:
http://www.videohelp.com/tools?tools...+or+List+tools