So, it works in 2k while set up as an ata-66 drive?
I had that error when moving from an ata-33 controller to an ata-100 controller in win2k. I'll go ahead and tell you how I fixed it, but it may not help your problem -
With the old controller installed and win2k working, go into the device manager and change the crontroller driver to "standard IDE controller". Shut down and boot with the new controller - it shouldn't give the "can't find boot device error" and will install new drivers for the new controller.