To be perfectly honest, I am extremely shocked that the update made it finally shut down without re-installing XP.
The ACPI interacts with the OS as well as other software to get them "familiar" with how to handle the hardware alligned with the motherboard. At first sight, since you mentioned that this didn't always happen, I would assume that a system change had been made recently to change this hardware/software interaction.
There are a lot of articles on this at MS's site and at other independant sites. Too many to link here but to learn more you can go to MS's knowledge base and type in some search criteria for ACPI and Windows XP.
I'm glad it's working!