suggest you de-install the modem from XP..than shutdown the PC and (if you have an internal modem, remove it physically as well) restart. Now the machine is without any modem.
If you have a USB modem you can plug it in now and XP will detect a new hardware device and will install the drivers.
If you have an internal modem, shutdown the PC again and install the modem physically. Start the PC and XP will install the new hardware.
This is also a problem with W2000 sometimes...
Good luck