Thanks to William C. Mott and Ramesh Viswanathan for the tip: Microsoft has posted a hot-fix that fixes a denial of service bug on NT machines with the Simple TCP-IP Services installed. A malicious attack may be mounted that consists of a flood of UDP datagrams sent to the subnet broadcast address with the destination port set to 19 and a spoofed source IP address. Windows NT computers running Simple TCP/IP services respond to each broadcast, creating a flood of UDP datagrams that can eventually crash the system. You can download the hot-fix on the Microsoft FTP site