An unidentiefied error

STOP: c000021a unknown Hard Error
Unknown Hard Error (on a blue screen)

and another reading:
Windows Security: winlogon.exe – system error

can anybody explain this error to me, i havent removed or damaged the drive. Scandisk picked up nothing, and upon reboot, the mouse response was normal, but once i click on any button, it takes atleast 5-10 mins to open. NB: I DO NOT WANT TO FORMAT, SINCE IM NOT SURE WHAT CAUSED THE PROBLEM.