Nero can not convert widescreen to fullscreen. There are very few people interested in doing that. You'll need to invest in a high-end editor that can mimic (or macro) a task and set it to crop the sides of every frame automatically, or spend an hour or so setting up VirtualDub filters into a 3-part script.
The easiest way to convert all your widescreen videos into antiquated full screen format is to buy a pan-n-scan capable DVD player. The majority of people just don't do things like this.
Please pardon my disgust. 8)