I do not think XP supports re-assigning IRQs.
However, you can often achieve a re-assignment by powering
off and changing the order of the boards in the PCI slots.
Also, the BIOS of some motherboards have a provision for
IRQ assignment. My ASUS P4S8X manual discussed the
possibilities, but fortunately the defaults worked fine,
so I left them alone. Consult your motherboard manual.
Finally, XP can support more than 15 IRQs. If supported
by the motherboard, XP should have automatically detected
this and installed the appropriate HAL. (Check your
motherboard manual.) If not, you can force the correct
choice during the XP installation by hitting F5 (not F6)
early in the installation process. I do not think you can
change this sort of thing after XP is installed, without
doing something damatic, like a repair installation. Even
that may not be enough.