DO NOT INSTALL EVERY CODEC KNOWN TO MAN....EVER!!!!!!!
Use FFDshow for video files (avi's), Videolan for DVD's (its a media player with a buit in MPEG2 decoder), AC3Filter for AC3 audio, a good MP3 codec (Lame or Fraunhoffer) and Ogg Vorbis/AAC/Monkey if required.
Grab either AVICodec or GSpot to check which codecs you still need (if any) to play anything the above does not cover. Also DUMP wmp and use a better media player...there are loads of better and free alternatives.
All named codecs are freely available.
You only really need FULL codecs when ENCODING so if your going to make an avi (for example) you will then download a version of the XVID (or DIVX if you like paying.....or the GAIN spyware on free version) codec and install that. FFDshow covers nearly all avi formats for DECODING.
There is also a freeware QTime player available if you use QT.