mine was actually the result of adware/malware.
open your task-manager/processes/show processes from all users. the malware on my computer wasnt running under my user, if i remember it was actually running under system. look around for something suspicious, google it, and if thats it before you end the process right click on it and open the file location. then end the process, and delete the files.
dont know if youll need a reboot or not after, i ran ccleaner on my registry and restarted. after that it booted up fine.
the adware i had was called something like savingsbull. i also had others to. hope this helps others