It is amazing to think that all of these things have happened to me since I started using Windows 7! I hope some ideas here help you solve your mysterious computer startups.
Mouse & Keyboard is Waking your Computer
Windows 7 has an annoying habit of starting up from a sleep or standby mode without any obvious reason. This list can hep you track down the cause, and how to stop it.
First, test to see if moving your mouse or hitting a keyboard key is the cause of the startup. For some stupid reason, every time you install a new mouse or keyboard in Win7, the default setting is to allow it to "wake from sleep". If this is the cause of the problem follow the steps below to turn it off.
Network Card wakes your Computer
The most common cause of a computer waking from sleep mode is the network card. Follow the instructions above, but select "Network Adapters" instead of Mice. Double click on your network card and proceed to the Power Management tab.
If the checkbox is already clear, then you might check a few other devices to see if they have a Power Management settings that is checked. If hardward devices are not causing the problem, then it must be an application (program).
Advanced Power Settings can allow your Computer to Wake
Windows 7 computers have terrific power options that allow you to put your computer into a standby mode to save power and enable fast restarts. Checking the Windows power settings - they may have been changed by recently installed software.
Automatic Switch from Sleep to Hibernate Mode
Does your computer "wake" from sleep and proceed to do a hibernate "save" to disk, then shut down again (in Hibernate mode, which uses no power and restores your system as it was when it went into Sleep mode). The Advanced Power settings is where you change the time before the computer automatically moves from sleep mode into hibernate mode - the default settings is too short. Under Sleep is a setting "Hibernate after", I set it to 2880 minutes (2 days) on battery and 7200 minutes (5 days) when plugged in. Another mystery startup solved!
Programs that Wake your Computer
As a last resort, you may use a small Windows tool called powercfg to discover what is waking your computer.
PDQ Library is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.