The everyday analog plug that connects most computers to monitors today doesn't support copy protection, and so is viewed as unsafe by Hollywood studios. Movies playing on a computer over this ordinary analog connection will likely be downgraded to near-DVD quality.
Even worse is the so-called DVI plug that sends high-quality digital signals to a monitor but also doesn't support copy protection.
That offers an even greater risk of copying in Hollywood's eyes. Studios have persuaded Microsoft to add a feature in the upcoming Vista operating system that can shut down that connection altogether, unless the computer has an Intel-created encryption technology called HDCP, or High-bandwidth Digital Content Protection, turned on to guard the signal all the way to the monitor screen.
Put another way--if the DVD doesn't like your plug, your monitor may go black.