Before you do that, realize Nero 8 will wipe out Nero 7. Make certain you have your serial number available as you may want to reinstall that.
Ultram: Your Nero 7 menu will look different from my screen shot. What I was trying to explain is that there is a point in your version where you need to select a target disc and select the size. What you select should match what you insert in your drive. I do not think it is necessary to install Nero 8 before trying my suggestion.
I am tying to Burn a DL DVD from a completion of a DVD Movie.
but more information would be helpful. What version Nero are you using and what Nero program?