Reboot the computer - right at the beginning (either the logo display, or some DOS-like text) you'll need to hit a certain key to go into the BIOS setup (it's usually Delete, if not that, try F1, F2, F8, F10, F11 or F12).  If done right, you'll be brought to a DOS-like screen with setup options.  There's usually a menu item with something to do with power, check in there and see if wake on keyboard/mouse is enabled.