From Seagate's web page:
What is required to run in Ultra ATA/100 Mode?
Similar to Ultra ATA/66, there are basically four requirements:
An Ultra ATA/100-capable system board and BIOS. (Ultra ATA/100 expansion cards are also available.)
An Ultra ATA-capable 40 pin, 80 conductor cable.
An operating system capable of DMA transfers, such as Windows 95 (OSR2), 98, and Windows Millennium Edition.
An Ultra ATA/100-capable device. (Seagate drives require an activation utility for each device.)
Where can I get the Seagate Ultra ATA/100 toggle utility?
The utility (UATA100d.exe) can be downloaded from our Drivers and Utilities page. The file is a self-extracting Zip archive that must be unzipped and placed on a bootable diskette. This version of the utility can be used with all Ultra ATA/33, Ultra ATA/66, and Ultra ATA/100 capable Seagate ATA hard drives