Was your modem supplied with your computer?
Were you running W95 or W98 previously?
If the answer to both the above questions is yes you may have to purchase a new modem.
Is the modem on the "Device Manager" list. If it is try installing the drivers for it over XP - that is if you have the original drivers on disc.
Go to My Computer - View System Information - Hardware - Device Manager - Modem, then right click Modem - click Properties - Drivers. Then try and istall your drivers from the disc.