|When you installed, upgraded or just opened WMP it asked what file types WMP will be the default player for. |
You should be able to undo that but you do need a player. If you have a different preference then installing it will usually make that player the default player. You need to read the screens and untick the options you don't wish to use.
Go to the start menu> default programs. You can find the options to reset the default programs, including WMP.