Sounds very much like an incompatibility issue. Are you sure your using the correct driver for Win XP. Even though Device Manager says its ok and working prop if the driver has loaded correctly it will not work. Do you know what make and model the modem is?? If you use a generic driver the modem will work but at a very slow rate. If you don't know take the modem card out of the PC and have a look BUT MAKE SURE your earthed before touching it..