Have you tried right clicking on them and selecting "open with..." Nero will be set as your default program for all music etc but you should be able to change it doing this, it should say do you want to make this your default program?
Try right click followed by "open with" selecting media player.
I believe there is also an option somewhere in the control panel to associate all that type of file to Windows Media Player.
I have tried *right clicking* but it does not give you the option OPEN WITH, but tried the control panel and it seems to have sorted it so a huge thanx to everyone !!