Cannot Remote Desktop to home PC after installing XP SP3

I know Remote Desktop isn't strictly speaking "Terminal Services", but it
seemed to be the closest match. After installing Windows XP Service Pack 3
on my home system, I cannot connect to it by Remote Desktop. The client seems
to be trying to connect for a moment, and then automatically cancels less
than a second later, with no error message (whereas I DO get an error if I
try to connect to the wrong machine or port). I have tried connecting from
outside my home network, from another PC on my network over the router's WAN
interface, from the other PC directly to the local network address, and from
the PC in question itself to the WAN interface (it won't let me try to
connect to its own local address); same result every time. Remote Desktop
worked fine before I installed SP3.

I have rebooted my system, disabled and re-enabled Remote Desktop in System
Properties, and added accounts explicitly to those allowed to logon over
Terminal Services using secpol.msc (they were already members of Remote
Desktop Users). I used to connect over a non-standard port for security
reasons, which was mapped to 3389 by the router. I changed it so the
non-standard port is passed straight through the router, and changed the PC's
RDP port in the registry; issue unchanged. Windows Firewall was and is
disabled.

Using Wireshark, I can confirm that the RDP client packets are reaching the
serving PC. If I post the capture here, will someone be able to look into it
and tell me what's wrong?
.

Relevant Pages

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