Serial.sys Issues Regarding My Modem

I am unable to install modem drivers on my client’s laptop. First I noticed that, under hidden devices in device manager, the Serial driver was in an error state (yellow !) so I found the driver in the system32/driver file and replaced it with from the XP Pro CD. This did nothing so I uninstalled it, replaced it again, and now I can’t even get a serial driver to appear in device manager. I have uninstalled and reinstalled the modem drive at least a dozen times. I’ve used sfc and I’ve even done a full-on repair of the OS . Any thoughts on how to get this driver back, running, and get this modem up and running? It’s a Gateway 200ARC. I downloaded the modem drive from gateway’s website.