Codec issues...

Ok, I'm pretty sure this is a codec problem, but I'm not quite sure how to nail it down. Whenever I open a folder that has an .avi or .mpg (and sometimes just image files) in it, Windows Explorer crashes. If I do not turn the message off, I can still watch the file, but as soon as I do accept the message, then it's all over. Explorer comes back up of its own accord most times, but about half the time I have to go in and shut down Dr. Watson with the task manager.
Anyone have any thoughts?
Anyone have any thoughts?