We also have a Lenovo B320 all-in-one, which we have had for about a week. We are having the same problem with the display going dark, but not as frequently as yours. We will try the solution of disabling the start-up programs except McAfee.
One semantic point. I believe you were not experiencing a sleep problem. If the computer goes to sleep, the light in the power on-off button will blink. If you touch the mouse or keyboard, it should wake up. In our case the computer, or actually just the display was freezing. I could still access the data on the B320 hard drive using the network connections to my other computer. So the B320 was still running, but the display would not respond. We have had to force a shut down by holding the on-off button in for a few seconds. So I believe you were experiencing a display freeze, which would not be corrected by changing the sleep timing.
At any rate, thanks for discovering the solution!