are you using the right driver?
The modem will use a different driver on a WinXP machine, and if you are using an old Alcatel CD it won't have it. Each Alcatel modem uses different drivers for different operting systems.
You can download the correct version for your model if you
I assume that the WinXP machine does recognise that there's a device there when you plug it in - although it doesn't necessarily know what it is?