Go to Control Panel and click on the QuickTime icon. Select the "File Types" tab and then unselect the file types you don't want QuickTime to be the default player. Suggest these are all MP3, JPEG GIF etc.
Don't have Nero but there must be a simiolar function to check and reset this as the default. Otherwise go to Folder Options in Control Panel and s]again select "File Types" tab. Here you will have to go through each file type extension e.g. JPEG and change Quicktime back to Nero.