Assuming that when you upgraded you had the Service Pack 1 revision to XP, the error on attempting to install the patch is correct.
The patch is for the pre SP1 release of XP, or systems that do not have SP1 installed.
As with any Windows shut-down issue, start with driver updates.
Video card,
Modem,
NIC
are all likely suspects.
Are you getting the old AT style "It is now safe to turn off your PC" screen?
If so, make sure that Power Management / ACPI is enabled in BIOS settings.
Then, enter device manager,
Click on view, and select view hidden devices / show all devices.
Right click on APM and choose enable.
Next, there is a very good XP shut-down trouble shooting guide posted at www.aumha.org
As a final side note, I have experienced a few hardware combinations that just would not shut down under XP.
On my first XP system, I had to change out both the modem and sound card to have reliable shut down operation.
Chas