When you're not a computer geek or Microsoft employee, you don't necessarily touch computers every day, and trying to remember which hot corner to touch or where your application is, or how to get out of a full screen Metro app, is not something they're going to remember or even want to try to figure out.
Then put stickers on the four corners of the monitor: "<- Start Screen" at the bottom left, "<- Switch App" at the top left, and "-> Charms" at the top right. It'd be like the cardboard overlays on the F keys back in the DOS days.