Some, but not all that many, users are having trouble with AMD/ATI's new Catalyst drivers for Radeon cards (version 7.3). The problem is apparently in hardware detection for hot-plugging displays and you needn't perform that action to encounter it. If you have a problem, the link above will take you to a decent workaround at Extreme Tech, along with "last resort" directions.
This workaround appears to be very effective. The one person I know who encountered the problem had it fixed by this trick. It's not too hard to do. Anyone can manage it with these clear and simple directions.
AMD/ATI will come out with a fix soon. Until then, use this.