In addition to what Mickey pointed out, RSLinx must not be running as a service to add the PIC driver, and you can't have windows XP SP3 for the OS. SP2 is fine. There is a workaround if you have SP3 installed.
Also, it's possible to change the firmware in the DH485 unit to make it a DF1 device. I just had to do this recently...it depends what your using it for whether DF1 can be used.