It could be your media ... to rule this out, try a good Ritek disk. These are renowned for their reliability and compatibility with standalone players and are more suitable for movies.

It's also possible that you are burning to the wrong format in Nero or your version of Nero is old, which may be causing your problem.

Try updating to the latest version of Nero, either v.5.5.10.50 or Nero 6. Then make sure you are burning, using the DVD-Video option.

If this doesn't help, post back again, giving more details of your method/procedure etc.