ACPI is a power management and resource allocater for your system. (Hope that is an acurate description) One of the things it does is allow more than one device to use the same irq. To disable it in Windows 2000 do the following.
Disable P&P OS and ACPI in your BIOS, if you can (you may not be able to, and it is not always necessary) - if the option is not available try installing the latest BIOS update for your motherboard. You can get help with finding out what BIOS you need and instructions as to how to flash it at Wim's BIOS page
Reboot. Go to Control Panel - Hardware and double click on My Computer in the hardware manager.
You'll see ACPI - get into its properties and click to install another driver. When prompted, look under the Microsoft section and install the "Standard PC" driver.
Reboot. At this point Win2k will try and reinstall all devices on your system so install any drivers you are prompted for (have them all handy) and reboot. Finalise any drivers you may be prompted for on the 2nd boot and reboot again.
With all your drivers re-installed, take a look in hardware manager - and every device should have its own IRQ.
Be warned that this method is discouraged by Microsoft - see the note at the bottom of this
http://support.microsoft.com/support/kb/articles/Q234/5/58.ASP article.