I have (3) BP6 systems running WInXP-Pro and all work OK wuth the RU standard BIOS. However, the v.1.25 Windows2000 driver is needed (on floppy) I believe, to make the install happy enough to complete.
Better idea is to go & get yourself a Promise ATA100 card (native support in WinXP) and leave the HPT366 idle. How big is the Hdisk anyway?