truerock

About 95% of the time, your problem is caused by a DVD CoDec that is incompatible with Windows Media Player 11 (or 10). WMP11 requires a special codec that will either support its copy-protection logic, or by-pass it. I don't use WMP11 because it doesn't play DVDs without a special codec, but I use a trial version of nVidia's PureVideo codec to test WMP11 from time to time. I think you would be better off not using WMP11. I have heard the Windows Vista Premium and Windows Vista Ultimate include the DVD codec - but, I haven't had the oportunity to test that rumour. All of my Vista computers (Dell and HP notebooks) have a 3rd party media player on them. It would be nice if there was a way to tell which CoDec WMP11 is trying to use, but that is not possible. I use Nero 9 Showtime to watch DVDs.

Many times it is recommended that a bundle of CoDecs be installed to fix WMP11. Personaly I would not do that. I recommend keeping you system as un-complex as possible. If you really want to fix WMP11 so that it will play DVDs, find a specific CoDec designed to fix it. Don't dump dozens of CoDecs into your computer.