This indicates an attack attempt to exploit a denial of service (DoS) vulnerability on Windows NT 4.0 systems running Point-To-Point Tunneling Protocol (PPTP) service.
An attacker can cause the target machine to reboot by sending a malformed PPTP control packet to the PPTP server.
Microsoft NT version 4.0 up to and including SP 4
Denial of Service: Remote attackers can crash vulnerable systems.
Apply appropriate patches or upgrade the system to the latest non-vulnerable version