I want to boot from a different SATA controller. Drivers for both
controllers are installed into Windows XP, and after physically
switching drives I can change the boot order in the BIOS, but when I try
to boot I just get "A disk read error occurred". Is there a document
somewhere on how to change your boot controller in XP?

It turns out that all my 32-bit disk utilities were lying, but Norton
Disk Edit for MS-DOS confirmed to me that the controllers were reporting
different geometry for the same drive.

Problem solved!

I had tried changing the geometry in the partition table and boot sector
to no avail. In fact, I could not remember how to change it back to get
the original controller to boot. So, seeing as this is a new PC, I ran
the system recovery procedure. And it recovered it using the new
geometry, which works using either controller.