Problems with PC Card modems in Windows 2000 : More Information
Firstly, open device manager and select the properties for the modem; Click on the resources tab:

Then click the 'Set Configuration Manually' button.

Uncheck the 'Use automatic settings' tick box. This will allow you to manually configure the modem.
From the drop down menu, select 'Basic Configuration 0004':

Highlight 'Input/Output range' and click the 'Change Settings..' button.

Select an appropriate I/O range that fits into your PCI bus range (such as the one shown above.) Click OK.
Highlight 'Interrupt Request' and click the 'Change Settings..' button.

Select any free Interrupt and click OK.
You should then see the new properties you have set for this device, similar to those below:

Click OK and confirm the assignment of properties as below:

By clicking on the 'Yes' button.
