USR modem install

I am trying to install a 3cp5610a USR modem in my Dell optiplexGX240 but after installing the modem into the PCI slot and booting, the computer finds the new hardware but refuses to recognize the driver (downloaded from the USR site). The error message states that it can't find the required drivers for the hardware installed. I am running windows 2000 pro sp4. I have tried having the drivers unzipped on the hard drive as well as on a floppy with the same results. Also tried installing the modem in different PCI slots. I did try this modem on a Dell running XP and will install fine.
Any suggestions? Thanks...