Some possible solutions are:

1. Open the Device Manager, click on the + sign next to 'Network Adapters', right click on Network Adapter and choose 'Properties'. Click on the 'Power Management' tab and uncheck 'Allow this device to wake the computer out of standby' or something similar.

2. Go to BIOS when your computer starts and disable Wake on Lan feature.

3. Also try to check scheduled tasks They could wake the computer from hibernation. You can find them at Start ->Accessories->System Tools/Scheduled Tasks