Knowingly or unknowingly, there's got to be a scheduled task going on. It could be an anti virus program, could be a disk back-up program, Windows scheduled update or even a virus, spyware adware planted on your pc without you knowing it.
Start your PC in Safe Mode, then do as Raptor suggested, run a complete virus and spyware adware scan first, then go to Add/Remove Program and find out if you have any disk back-up and/or anti-virus programs installed, if so go to the program to change the scheduled time or simply uninstall them for now (you can always reinstall them later.). clean the registry then restart your pc again in Safe Mode.
MacPC