Why can't I play a video dvd at my dvd player?

You may have used the wrong app to burn the disc. "Nero Vision" is what you use to burn a DVD. It will automatically adjust the aspect ratio (or you can choose your own), option for PAL or NTSC, and will convert into standard DVD format. The write-speed is also important - choose the lowest, usually 4X, there'll be less chance for errors, and disable any programs that may interfere while Nero is running, i.e. screensaver, autoupdaters, etc.