Because that is an NT error num (see http://msdn.microsoft.com/en-us/library/cc704588%28v=prot.10%29.aspx http://msdn.microsoft.com/en-us/library/cc231200%28v=prot.10%29 ) not a Win32 error num (http://msdn.microsoft.com/en-us/library/windows/desktop/ms681381%28v=vs.85%29.aspx ). In Win32-ese 0xC0000005 would be 998 ERROR_NOACCESS http://support.microsoft.com/kb/113996.