I just added my first USB device to my Windows 2K Pro machine, an Iomega Zip Drive 250. WHile the drive does work (sort of), I get messages every few minutes that I've unplugged the device (which I haven't). I called IOMEGA tech support and, after walking through several steps they told me that my problem was that the USB was on the same IRQ (9) as another device (my video card). I cannot see any way to assign either of these two devices to another IRQ. I've uninstalled them both and let Plug n Pray discover and reinstall them many times with the same result. I've had the same problem (constant 'unplugging') with a USB scanner I tried as well so I don't suspect the problem to be with the Zip drive itself.
I've gotten and installed the latest drivers from IOMEGA and am completely up to date on Oper Sys updates.

Any ideas how I can get the USB Bus assigned to another, open IRQ?

I have the same exact problem except with an Iomega Zip CD. Its ACPI problem. It assigns all your devices to the same IRQ which really sucks. I have no idea how to fix it and its a major problem. I thinking about going to an older version of windows to fix this problem. Anyone have any ideas how to set irqs manually when acpi is working?

I finally broke down and called Microsoft. They don't know either. Basically it depends on the deivces that Plug & Pray attaches to the PnP interrupt (9). In my case it was the Video Card and USB ports.
MS answer was to diable the on board video, purchase a new video card that was not Pnp (imagine a video card with Win2K drivers that is NOT PnP!!!!). Then make sure that the new Video card was configured to another open IRQ.
At least they didn't charge me for that horse$hit solution!.

On new motherboards that are ACPI compliant, win2k will share PnP devices on IRQ 9 deliberately. This behaviour is due to the design of the ACPI interface. The MS knowledge base covers EXACTLY this issue in some detail (search knowledge base for "ACPI" or "IRQ sharing").

ACPI is meant to FIX rather than cause IRQ sharing woes... as long as the device supports IRQ sharing. This sharing probably means performance is sacrificed to some degree.

My machine uses a 250 Zip on USB with no problems at all (I have 8 devices sharing IRQ 9). I have other free IRQs that ACPI doesn't use - the knowledge base article discusses "resource rebalancing" too. Make sure you have win2k SP1 installed then check "windows update" for additional minor fixes. By the way - I have not installed any iomega drivers so they might be a problem. Also most ACPI motherboards recommend disabling PnP support in bios since ACPI covers this function.

Hope that helps.