Windows Vista, by default, goes into sleep mode instead of powering off.
A nice functionality, sometimes I don’t reboot my PC for weeks.

However, when I sleep my Vista and then unplug my USB mouse, the machine wakes up again. This can be anoying when you want to leave. A workaround is to unplug your USB mouse before sleeping the machine 🙂

A better solution is this:
1) Check which devices are capable of waking up your machine when sleeping by opening a command prompt and run the following command:

powercfg /devicequery wake_programmable

You might get something like this:

Logitech USB iFeel Mouse (002)
Intel(R) 82566MM Gigabit Network Connection
Intel(R) Wireless WiFi Link 4965AGN
 AuthenTec Inc. AES2501A

Then open to device manager, go to the device you think is causing the wake up (my mouse in this example), double click it and choose power management.

Disable the checkbox “Allow this device to wake the computer”