These kinds of hassles can happen after a period of “idleness,” when the system detects no mouse or keyboard input. Blame Windows’ power settings, which by default try to conserve power if they think you’re not doing something. (Funny how Windows isn’t yet smart enough to know if a movie’s playing or there’s a projector connected to your laptop.)
Most modern video players will disable the screen saver automatically, but a few have yet to gain this feature (I'm talking to you, HTML5-powered YouTube). Furthermore, if you're trying to do some reading, it can get annoying to have to jiggle the mouse yourself every few minutes to stop your screen saver or un-dim your laptop's monitor. Mouse Jiggler is a tiny program that runs on your desktop and subtly jiggles your mouse to keep your computer awake.