Your computer is restarting due to a power failure, and you have your computer set to come on when power is resumed.
Go into your CMOS setup and search for this setting. There may be three options: off, previous state, and on. "Previous State" should mean that if there's a power failure while your computer is shut down, it won't wake up. This may not work. If you set this setting to "off" you can be sure your computer won't "wake up" after a power outage.
You have an application that has scheduled your computer to wake up at a certain time.
Some applications can do this - such as the windows task scheduler. Even if this feature is disabled in your CMOS setup, windows applications can override this because they can use the APM or ACPI power interface.
You have a Wake-On-Lan network card or a Wake-On-Modem modem, and you receive a wakeup signal in either, and you have this feature enabled in your CMOS setup.
1. Make sure that the computer si actually shutting down and not stopping half-way there.
2. Check the BIOS. I have my computer set to start at 4am and this option is located in your BIOS before you even start windows. Now, I have never checked to see if this option is in windows itself, but check you energy saver options and power options in the control panel.