I noticed I had the same problem that just started today. I read some comments on other sites where they found the answer to their problem in the event log, under system. When I looked under sleep, it said that a USB root awakened the system.
Well it just so happened that I just bought a new wireless keyboard/mouse combo and installed it yesterday. It appears that the keyboard and mouse were still sending connection signals to the receiver which would make the machine awaken before it would completely go to sleep.
So I went into the device manager and disabled the mouse and the keyboard from awakening the system, and bingo, it went to sleep (originally I did just the mouse and not the keyboard). The only downside apparently is that you have to use the power button to wake it up. In case you dont know how, here's how:
1. Go to Control Panel
2. Click on System
3. Click on Device Manager on the left of the screen
4. Expand on “Mice and other pointing devices”
5. Double click on your device
6. Click the tab that says Power Management
7. Uncheck “Allow this device to wake the computer.”
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