Did you try assigning other drive letters to what's working before plugging in the next drive?
I'll not write how as XP has been out for over 11 years.
I have an Epson DiscProducer PP-100 which has two DVD-R drives connected to my computer with one USB 2.0 cable and conflicts with a Kingston DataTraveler 4GB USB 2.0. I know this because the Kingston drive is not recognized whenever it is inserted with the discproducer is on and its USB inserted. I know the obvious solution is to eject the DiscProducer but I often need to use both products simultaneously.\
I have a Dell Optiplex GX620. The OS is Windows XP with Service Pack 3. The processor is a Pentium 4 CPU 3.40GHz; 3.39GHz and 2GB of RAM. My keyboard and mouse also have USB connections and the computer tower is plugged into a four port KVM switch.
Thanks for your help!