Thank you for the helpful advice; but I have already managed to find, download, and install "SP4". In fact, I installed SP4 prior to downloading the WSUS XP updates that were kindly available on this site.
I'm not sure how much any of it actually helps, though! Considering how few people run XP now, and that almost certainly no one is writing viruses and malware for it, I wonder if the WSUS and SP4 security updates are really necessary.