I have a Motorola SM56 Data/Fax Modem on a PCI slot and it works fine . I used the Windows 2000 drivers available for it. Try installing the Win2k drivers and ignore any warnings which say that this driver is unsigned and shouldnt be installed.
if the drivers are on a cd then intead of running any type of install, do an update drivers from device manager and let xp search the disk for the driver it needs, find this works more often then the written installs.
if not then there other ways force loading them