USB Human Interface Device support at boot time
I think you have been offered all of the viable suggestions for using a USB keyboard at boot time. You say that it works in the BIOS and that USB Legacy support is enabled but the Windows XP SP2 installer still doesn't recognise the keyboard.
Is the keyboard plugged directly into a motherboard USB port rather than an add-on card or external hub?
Could it be that your hardware, specifically the USB ports, post date support in Windows XP SP2? In other words, have you got USB 2.x or 3.x ports that SP2 might not detect correctly?
I doubt that the keyboard itself is the issue as there should be generic support for Human Interface Devices which enables the basic functionality even if all of the multi-media keys and extra mouse buttons don't work and, after all, it is working in the BIOS.
Now for the frustrating question: Do you have access to any other bootable OS to test the keyboard with? Can you borrow an XP SP3 disk, a Linux CD/DVD or even a DOS floppy?