Have you checked what is waking up your system? Try the tips here http://www.cravingtech.com/fix-windows-vista7-sleep-mode-from-waking-up-by-itself.html
I usually find it is the mouse, as a small vibration can cause the mouse to register a movement and so wake the computer. If the computer is asleep and you move the mouse, does it come out of sleep, if yes disabling "Allow this device to wake the computer" from all the Mice and HID devices in Device Manager is a good place to start.
The wake timer option is only for timer settings, so that shows it isn't a timer waking your system.
After testing for several times, I found the reason eventually.
"Wake on Patten Match"!!!
This option almostly let me exhausted, and I found many people had the same nightmare with me
In the Windows 8 release preview there doesn't seem to be a way to disable wake on pattern matching. The default MS drivers do not include wake on LAN options so I installed the latest Intel PROSet drivers using compatibility mode. Even after disabling everything but wake on magic packet, the DH77EB board I'm using immediately starts back up after shutdown. The only way I've found to stop this is to go into BIOS and disable wake on LAN.
Recently a "login windows" always appears while I am visiting a certain website http://ckhung0.blogspot.com/
I do not know what this is mean and if this will do harm to my system.
Anyone who knows please tell me about it, thanks.