To get ATA100 performance, you need three things: both the disk itself and its controller need to work that way, PLUS you must have the proper ribbon cable. The original ATA ribbon cables had 40 wires in them with 40-pin connectors. But a later better design uses 80-wire cables, still with the same 40-pin connectors on the ends. In that design, every second wire is a ground lead and it prevents signals from mixing between the lines. Cleaner signals allow the system to run faster. So, look closely at the ribbon cable and start counting over from one edge. If you get to 10 wires and you're already 1/4 across the ribbon, you have the older style, so replace it.
Other than that, BIOS settings can play a role. But they don't often say ATA100; instead they will talk about PIO modes, etc. The higher the better, but your system will limit you on how high you can set it. Part of the limit is imposed by the measured capabilities of your system, so make sure you're using an 80-conductor cable before trying to change this.
Another small point. ATA ribbon cable standards limit the cable length to 18" tops, so if you are using a cable longer than this it MAY limit your speed. I've been getting away with 24" cables for a while, but that does not always work.