One of my friends had a similar issue, although it wasnt Vista, it was XP. His solution was to not use Windows Media Player for watching DVD movies, so he installed a DVD Movie Player off of a DVD that came with one bundled with it. He then bought an external DVD-RW drive and that came bundled with better software called WinDVD Player.
I use PowerDVD on my Windows 7 system myself vs Windows Media Player for watching movies. I got this software bundled with a new DVD-RW internal drive that I bought when i upgraded from IDE to SATA for my optical drive for faster DVD Burns.