Windows XP doesnt let you assign IRQs.
Only safe way is to reinstall and use a non-ACPI configuration.
http://support.microsoft.com/kb/299340/EN-US/
You can also google ACPI, HAL, IRQ etc and see what experiences people made with it. Generally ACPI works well - even if a lot of devices are assigned to the same IRQ. After extensive reading i have to conclude though that even switching to non ACPI didnt resolve issues for most people because the problems were related to other issues.