Can you help me in the fact that wmv is giving me sound but no picture. This is an avi file I have downloaded that will not play, is there a reason for this?

I am really starting to get annoyed as all I want to do is download some eps and I keep getting new things thrown at me when I followed the so called easy step by step guide, is it something I am missing on my computer? I do not know, please help as I am a bit of a novice and need dummy guides.

Terri