I downloaded an avi file and when I tried to watch it in windows media player it said "error downloading codec." I found out that all codecs have a four letter code. The easiest way to find out what the code for a file is to open it in notepad, and look for it on the first few lines. I realized mine was DX50. After googling for info on DX50, I downloaded the codec from http://www.divx.com/divx. I had to scroll down a little to find the download for just the codec, not the viewers that divx offers.