It has been a very long time since I used Nero 6, but it sounds like you are following the correct procedure. My suggestion would be to uninstall Nero, use the Nero clean tool to remove any traces of the program from the registry, then reinstall the program. You can find the clean tool here: http://www.nero.com/enu/support-nero6-tools-utilities.html
If that doesn't work, ImgBurn is darn near bullet proof. You probably won't like the interface, but it just works. It is also free to use. www.imgburn.com
Here is the guide for burning dvd-video to a single layer dvd with ImgBurn: http://forum.imgburn.com/index.php?showtopic=4632
By the way, when burning dvd video, you need the entire structure, which means you have to have the .ifo, .bup and .vob files within a Video_TS folder. Use the folder as the input.