That would be as lightweight as it gets (memory footprint of both programs: around 5MB):
Nirsoft's CurrPorts is
network monitoring software that
displays the list of all currently
opened TCP/IP and UDP ports on your
local computer. For each port in the
list, information about the process
that opened the port is also
displayed, including the process name,
full path of the process, version
information of the process (product
name, file description, and so on),
the time that the process was created,
and the user that created it.
In addition, CurrPorts allows you to
close unwanted TCP connections, kill
the process that opened the ports, and
save the TCP/UDP ports information to
HTML file , XML file, or to
tab-delimited text file.
CurrPorts also automatically mark with
pink color suspicious TCP/UDP ports
owned by unidentified applications
(Applications without version
information and icons)
CurrPorts is freeware and portable (no installation required).
If you're looking for a 'real' firewall application for XP, have a look at iSafer (a portable version is available here).
iSafer is free open source freeware.