According to Microsoft, this happens when someone tries to login into their account, and Windows is busy scanning the PC. It interrupts the process and causes corruption. Another reason could be a sudden OS crash or abrupt system shutdown while a process is running in the background. Some significant reasons have been known to occur as a result of the following: Compromised system or user files Windows Crash or BSOD Failed Windows Updates Missing User Files Outdated Windows Drivers Corrupted user account active directory. Power outage during the Windows update.  Damaged hard drive file system caused by disk write errors, power outages, or virus attacks. The Steam folder is write-protected. Failed Automatic Updates to Windows (which involves upgrading service pack installations) Low Storage Space