First, WMP on you X50 will not play .avi files. One of the short comings of the program. Second, do you have these files in a My Documents folder? That's how I have mine setup and everything is found just fine. Example: SD Card/My Documents/File.wmv.
Last, I recommend that you get betaplayer from here:
http://betaplayer.corecodec.org/ . It is the best video player around.