Core security functionalities of your operating system are broken or corrupted. Please run an anti-virus scan and check your system for corruptions and possibly repair it by following these guides: http://support.microsoft.com/kb/929833 and http://support.microsoft.com/kb/947821. If that doesn’t help please perform a repair reinstallation of your system.
This answer is copied from BE's FAQ and slightly adapted to be Unturned-specific. Click here to visit the BE support page.