Bilbo09 likes to hibernate his computer at the end of a day's work. He asked the Answer Line forum what keeps waking it up after a few hours.
I strongly believe in hibernating your PC. It saves as much power as turning it off, but when you reboot, it comes up faster and returns to where you left it. But those advantages are meaningless if it doesn't work.
It's possible that something on your network is waking up the PC. You can fix that in Device Manager: In XP, select Start, right-click My Computer and select Properties. Click the Hardware tab, then the Device Manager button. In Vista, click Start, type device manager, and press ENTER. (Vista has its good points.)
Follow these instructions:
1) Double-click the Network adapters heading to expand it.
2) Right-click your adapter and select Properties.
3) Click the Power Management tab.
4) Uncheck Allow this device to wake the computer.
If that doesn't work, and you can't find another cause, there's a simple workaround: After you've hibernated your PC, deprive it of electricity. At least, it's simple with a desktop. Just unplug the PC or turn off the surge protector. For a laptop, you'll also have to remove the battery.
My thanks to Tech4me for his very useful contribution in the original discussion.