I heard that OS X Mountain Lion on a Mac has a feature called Power Nap :
With Power Nap, your Mac sleeps but your applications stay up to date. So you have the latest information — such as mail, notes, reminders, and messages — when your Mac wakes up. Power Nap performs Time Machine backups to Time Capsule and downloads OS X software updates while your Mac sleeps, so you can begin installing as soon as you wake it up.
Can Windows do the same on a PC?
Does Windows 8 / Windows RT have any new features that can do any special task while it is sleeping?