What is keeping my PC from sleeping?

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

1. Open an elevated command prompt
2. 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.