I've had Roxio mess up a system before (freezing it, not restarting it, however). I'd remove Roxio before doing anything else. Then try watching DVD's with Windows Media Player or Media Player Classic.
Did you upgrade the video card (or overclock it)? I thought I read that some Dell's (probably older ones) were having a problem with certain video cards.
In the mean time you can turn off the screen saver. Modern monitors don't need them, esp. LCD's.
But then again I don't have any experience with Vista so I am just throwing some things out there.