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Windows XP is designed to automatically install drivers for devices like modems - but doesn't always do this correctly.
Modems that require operating-system dependant drivers (controllerless with DSP, or software / HSP) require an XP-compatible driver.
Microsoft also has a driver signing policy: when XP detects new hardware, the default policy "...installs the driver that is the closest match for the hardware, whether or not that driver is signed; however, given drivers of equal rank, the system installs the signed driver rather than the unsigned driver."
Once a driver is installed, the policy determines what happens when you attempt to upgrade or change the driver.
Your XP's system policy for driver signing can be reached from the Driver Signing button of Control Panel->System Properties->Hardware
The options here allow you to ignore and install unsigned drivers, warn, or block. If you are logged on as an administrator, you can make one of these choices the system default.
Many good modem drivers are unsigned - I do not recommend blocking unsigned drivers. Make sure your XP system is set at either Warn or Ignore. If the setting is Warn, you'll get a dire warning when you attempt to install/upgrade an unsigned driver. With modem drivers from a trusted source, you should accept the installation of unsigned drivers. (I also recommend backing up the registry before and after installing drivers. (This can be done with the Microsoft Windows Backup Utility by backing up at least the 'system state'.)
More information on Microsoft Driver Signing from Microsoft.