After installing Nero, close it.
Open Media Player and then Tools, Options, File Types to re-associate the files that Nero stole.
I believe you can avoid this from happening within the beginning of the Nero install. Each one of the features to be installed will offer some kind of Custom installation button. You should always customise your installs to see what is being installed on your computer along with the intended software. Sometimes unnecessary programming is installed. (This is how folks get 3 versions of Acrobat Reader on their machines.) During one of the feature installs of the suite you'll get the chance to deselect the file types that you don't want changed from Media Player and other programs.