Have you disabled serial ports if you are not using them? That will free up some IRQs.
All devices on 1 IRQ huh? Windows 2000 has been installed in ACPI mode and that is how it assigns its plug and play resources. Most people have no problems, other people do have problems. Usually it's all assigned to IRQ9...supposedly it's used as a gateway to virtual IRQs above 15.
Anyway,
here is the article from Microsoft dealing with it.
If you want to manually assign IRQs, or just let Windows do it properly, you have to change the ACPI mode HAL to Standard PC HAL.
To do this, go into Device Manager, and look under Computer. It should say Advanced Configuration Power Interface PC. Change this to Standard PC by changing the driver. Then reboot. Hopefully it should come out OK. The problem with doing this is that it does not support the APM drivers as a fallback, so you will need to manually switch off your computer. There is a way to install it....it's here on the forums somewhere, although I do not believe it is a complete solution.
If you need to reinstall, hit F5 while it is detecting all the devices right at the very beginning to manually select the Standard PC HAL. You may also need to turn ACPI off in the BIOS if you have that option.