I'm not sure if this will help, but you could try going into Internet Options and disabling the 'play in window' feature. Probably totally unrelated. If you want to do away with the left sidebar completely, then go into Folder Options, General Tab, and select Classic Folder View.
The tasks sidebar can be manipulated by any program that cares to take advantage of its interface, so another program could possibly take WMP's place there, but I'm not sure where you'd find one.
EDIT: Oops, I thought you were using Windows XP. I haven't heard of this before, could you post a screenshot or something?