I'm surprised this problem isn't better documented. I've had precisely the same troubles as aegris, albiet with no happy ending.
I run an Asus A7V, and a Maxtor ATA-100 drive. I am fully aware of the boot device driver portion of the win2k setup and have had success using it with the external Promise ATA-100 adapter in the same machine. However, the on-board device (which is theoretically precicely the same - even uses the same driver) will not install correctly under win2k setup.
Specifically, it prompts for the driver (after hitting F5 or F6.. it's been a while) and I go ahead and provide it. It accepts it, and everything seems fine until it's time to reboot. Blue screen - every time. "Inaccessible Boot device" I think is the message.
Aegris does not have a damaged/faulty motherboard. I've spent weeks on this problem, finally accepting that I'd have to continue to use my PCI adapter instead of the on-board. However, this isn't an option now, because of all of my other devices which seem to conflict with the external adapter.
I've been assuming that this problem will be rectified with a firmware update to either the promise BIOS, or the Asus BIOS or a new win2k compatible driver.
Incedentally, the aforementioned solution did not work for me. I tried installing win2k on the regular ata-66 bus, then installing the ata-100 drivers, then switching the cable to the ata-100 bus, and predictably, I was met with the same blue screen as in the installation of win2k.
Any new developments - I could really use it, asWin98 is just getting too unstable for my liking!
Thanks all