Since no one really seemed to answer your question, what you need to do is download the newest Promise ATA100 drivers from ASUS's site (last time I checked Promise did not have drivers for it on their site). You're going to want to unzip the drivers zip file you download to a floppy disk (yeah, I know... a friggin floppy disk). During the startup of the 2k installation, press F6 (not F8, it does say F6... I think someone pointed that out already). It will ask if you want to specify other drivers and you do. Point it to A:\ and it should find them.
The other way to go about it is to bypass the ATA100 controller for now by installing with the hard drive(s) hooked up to the ATA66 controller. After the installation is done, install the Promise ATA100 drivers in the normal way, and then switch your drives over the ATA100 controller.
I have only tried the first way, but I see no reason why the second way would not work.