What is keeping my PC from sleeping?
How can I find out why my laptop or desktop won't automatically go to sleep
I believe that Windows 7 is the only version of Windows which can tell you what devices are preventing the machine from sleeping. Windows 7 and Vista can tell you what devices are armed to wake the device from sleep.
There is a tool called powercfg which can do this. New to Windows 7 is the ability to see what is armed.
Windows 7
- Open an elevated command prompt
- Type powercfg /requests
You will see a list of items preventing the machine from entering sleep.
Vista also has powercfg.exe, which can tell you which devices are armed to wake the system from sleep:
powercfg -devicequery wake_armed
as well as which device last woke the machine from sleep:
powercfg -lastwake
Sounds like Windows 7 got some new features though.