ATA100 works in W2k and in XP. Keeo in mind that you have to supply the drivers for the ATA100 controller during the installation.
W2k and XP treats it as a SCSI controller.

I'm aware of the drivers being loaded during installation...
 
ATA-100 did not work properly in Win2k. Best you could do was 66.

well it really depends on what type of controller you have like if it is an add on card or and additional set of controllers on your mobo then it usually sees the drives as scsi devices which removes the bandwith limitation. However if you have an ATA100 controller that is a primary controler on the mobo (integrated into the north bridge) then you need a hot fix for win 2k to get ata mode 5 enabled. Win XP has all the sp2 bug fixes and add ons built right in including native support for ata mode 5.

I'm currently running ATA100 in Win2K Pro, but not after a couple hours searching for the aforementioned hotfix. It was posted on the main page of this site a while back. Went from serious buffer underruns at 12x burning to speeds that would support 40x. Nice.....