This is normal in W2K and XP. It is a result of the ACPI specification. http://www.acpi.info/index.html

ACPI is, among other things, a solution for the limited number of IRQ addresses available in the original PC design. It works by switching a single IRQ among a number of devices as needed transparently. I would not try to outsmart ACPI, it is a valuable fix that allows PC's to meet modern computing needs. It can be turned off in the BIOS, if there is a problem with your system, but if it is working you should leave it alone.