|Can't you download any necessary drivers using the computer you're on now?|
Are there 2 actual modems in the computer or just one? The '2 phone lines on back' may be the line (for the wall jack) and the second a connection for an optional phone as most modems have both.
If you have one modem then uninstall both from device manager. Then reboot and install drivers, if necessary.
If you have two modems, well, why? For some modems there's software to link the two for faster downloads but otherwise you shouldn't need it. In that case uninstall both from device manager, shut down and open the case and remove one of them. Then reboot and install drivers for the remaining one, if necessary.