Hi everyone, I've searched for an answer to this question but can't find the answer, so if the answer is here somewhere I'm sorry for asking about it again!

I've been downloading a few video files recently, and for some reason some avi files play perfectly ok, but other times I get a message in Windows Media Player saying that I either don't have the right codec. It doesn't tell me which Codec I need! Can anyone help?
Thanks in advance for any advice.