Realtek On-board Ethernet Controller is invisible to Windows 7

For some reason, the Ethernet controller on my Mom's computer is not visible to the operating system at all. It's been like that for 3 years, and she's been using WiFi as a substitute. The warranty on the machine is already expired and she should've RMAed it then, but Moms. The problem is especially aggravating because a special Ethernet cable that's 100 miles long was purchased and installed so that she could connect directly to the router. There are no references to the Ethernet controller in Device Manager at all, and attempting to install the driver using the automated installation program from the Dell website results in an error saying that the controller was not detected, and the installer undos its work. I also tried the Add Legacy Hardware option in Device Manager to install the driver by  clicking the Have Disk button and selecting the INF included with the installer, but nothing shows up in the window where you select the driver to install so I couldn't click Next. Someone suggested to unplug the computer, remove the CMOS battery, hold the power button, and whatnot, but that did nothing. Someone else suggested to disable the on-board Ethernet controller in the BIOS, boot, then go back to the BIOS, enable it, and boot again, but that still did not make a difference. It's not a big issue since the WiFi speed is still decent, but it would be best if it worked. The computer was never upgraded and still has the stock operating system.

Computer:

Dell XPS