I'm not extremely familiar with Vista, but it could be possible that your Operating system became corrupted somehow.
Do you have any recent restore points that you can load to? A restore point is basically a point in your systems history that can be loaded to eliminate any irregular changes made since said restore point was created. Think of it as a type of "reset to earlier time" thing.
Windows media player can be a very... difficult program, depending on your computer. It is hardware intensive and sucks a lot of RAM.
If all else fails and you're still experiencing these issues, back-up your data, and then run the Vista system repair utility.
The below link has a tutorial about how to do this.