This same problem was posted here by another member and I read the other thread responses, but no answer was discovered. I am having the same issue the system boots on its own every night. I have done all the scheduled task checks and disabled everything I could find, but it still turns itself on nightly.
I have checked the BIOS to no avail can't find anything there either. Looking for help. Thanks in advance!
If you put the machine to SLEEP when you go home, then it WILL be capable of getting awakened by scheduled tasks, including (as has been mentioned) Windows Media Center nightly GUIDE updates. But this requires that you've configured and activated WMC and are actually using it for TV, where the GUIDE updates are required and normally performed each night at around 3AM.
There are also other standard Windows functions and services (e.g. defrag, Windows updates and auto-restart, etc.) which also are automatically scheduled as part of a new "retail" Windows install, and must be configured by you to disable them or configure them to your liking. Again, when the machine is in SLEEP mode these types of services can wake the machine up.
Same with other software products (e.g. anti-virus), which also may do daily periodic updates, and might be configured and able to bring the machine out of SLEEP state.
So, you might try NOT putting the machine into SLEEP state when you go home for the night. Instead you might try using HIBERNATE (requires enabling in Windows, as it is not enabled by default). Then put it into that state when you go home for the night. In HIBERNATE state it is much less likely or impossible to awake from without your manual push of the POWER button on the case, or a button or mouse-click or keyboard-touch (if your BIOS can wake the machine that way) the next morning.
Or... just SHUT DOWN. Now it absolutely cannot be automatically powered up except by you, manually doing what you need to.
I don't think the BIOS of the machine can do an auto-power-on and self-initiate the boot process, but check you motherboard's manual to see if it is the BIOS itself which is currently awakening your machine from whatever "soft-OFF" state you are placing it in when you go home for the night.