if NerÃ¶'s installer behaves like the older versions, then you should be theoretically able to select the file associations if you chose the “Custom Install” option. Hopefully they still offer this option.
Actually there is nothing of the sort giving one option to set file association. I only noticed this after installation and searching the program itself found not such settings. So as I mentioned what I did before was install Nero 9 reloaded and made a print out of the associations it stole and then reimage my working previous image without nero 9 and now when I reinstall Nero 9 I can find and forced back the file associations back to what they were set at default to. This is the only way I can see that will put back the right track.