Download WPCREdit and WPCRSet. Edit is for on-the-fly-testing, Set for loading a driver at startup that sets the desired values.
Edit will automatically choose the correct configuration for a BP6 (IIRC). You can edit the registers in a binary/hex editor. Then you start "Installdd" that comes with Set and installs the device driver. Enter the values you changed and save them, so they´ll be restored after reboot.
If your system hangs at bootup, you entered the wrong values... but you still can uninstall the driver in safe mode.
BP6, RU BIOS, XP SP3, ACPI, 2x366@523(1,95V), Pentalpha HS + 1x 12cm fan @5V, 768MB, Powercolor Geforce 3, RTL8139D NIC, Terratec EWS64L, Samsung M40 80GB (2,5''), LiteOn CDRW