At the beginning on the setup (booting off the CD), if gives you a prompt to "PRESS F6 To specify a SCSI controller or third party driver" Your ATA/100 is NOT a native IDE driver, so you must treat it like a scsi drive, and install the driver manually prior to installation.
Here's the note under January Technical Column on the Asus website:
Question:
I cannot install Windows NT / Windows 2000 on A7V or CUBX-E board when the HDD is connected to the Promise ATA100 IDE connector?
Answer:
Please follow the procedures listed below for Windows NT/2000 installation.
Copy complete contents of Promise directory from ASUS support CD or web download to an empty diskette. Contents should include the OS folders and files txtsetup.oem and ultra on the root directory of the diskette.
Please change boot sequence to match your Microsoft install media provided. Proceed with the Microsoft setup process until you reach the "setup is inspecting your hardware configuration." Please depress the function key F6 until you see the "Mass Storage Controller auto detect screen"
Next a screen will appear showing a list of Mass Storage Devices (IDE Controllers, SCSI cards, Etc. that have been recognized by the setup program. Press the "S" key to specify additional Mass Storage Controllers
When you reach the next screen, "Other (Requires disk provided by a hardware manufacture)" will already be selected on the bottom half of the setup screen. Press the "enter" key to continue.
You will now be prompted to "Please insert the disk labeled manufacture supplied support disk" Insert the promise driver disk you created and then press the "enter" key when ready.
Now setup should display the following: "Setup has recognized the following Mass Storage Controller devices in your computer", "Promise Ultra100 Controller". Press the "enter" key to accept.
Continue on with the setup process until you reach the screen that prompts you to select a partition in which Windows NT will be installed. Select the partition type and press the "enter" key to continue on with the NT setup process.