adicoto's
list of codecs is a great point to start from ;)
Anyway for noob users sometimes is simpler to use a codec pack...
99% of times I (and others on this forum) discourage to use codec packs because really often they produce worst results (when they produce some results :lol: ) than to use only the needded codecs.
I suggest you to try to install FFDShow if you want something that will decode almost all the video types, then use GSpot to know which codecs you need to decode the unplayable movies and refere to adicoto's list to know where to bring them.
Finally, if you are really desperate and can't play the majority of your files, try to install K-Lite Mega Codec Pack ( :twisted: ), yes, one of the evil codec packs I spoken of above.. Install it with the Playback profile, and instead of using Haali splitter, change the corresponding fields with Gabest's alternatives.
TRY IT ONLY IF THE PREVIOUS METHODS DON'T WORK, OR IF YOU KNOW WHAT YOU ARE GOING TO INSTALL ;)