Hey I am really new to this to but I had the same problem, all it was for me was the player I had didn't have the right codec. So I just down loaded another one from www.divx.com and it worked first time. dont know if will help but try it.
Download GSPOT and load the file into the program. It will tell you which audio or video codecs you will need. Good Luck.
Its not a good idea to install everything from codec packs, just take what you need and leave what you dont. This is just a precaution against future problems like codec conflicts.
yes stay clear of codec packs, also i would suhggest getting a copy of avicodec aswell as gspot, i find it better in many ways!