I couldn't find out any information on how this was hacked. I assume that a binary differ was used against the orginal tcpip.sys vs the patched one. Halvar Flake's bindiff plugin with IDA-Pro, quite probably.
However, after analyzing the fix, I found it instructive to disassemble tcpip.sys to find out what the hack accomplished.
Basically, tcpip.sys (XP SP2) introduces a pointer value in the image that points to the maximum number of concurrent connections. In a couple of places in the code a check is made to see if the current number of connections exceed this value.
The maximum number of connections is set with this instruction:
mov dword ptr [_ActiveOpenProgressThreshold], 0Ah
The hack changes this to:
mov dword ptr [_ActiveOpenProgressThreshold], 0FFFFFEh
The disassembly makes more sense if you have symbols for tcpip.sys. You can obtain this by setting the symbol path to Microsoft public symbol servers: