Depends on the computer. Many nowadays are built into the motherboard. You'll have to go into "control panel" or whatever Microshi$ calls it nowadays and disable it, then install your new one. Depending on your computer and how "generic" it is, an off the shelf ethernet card may work, or you can get them that plug into an USB port as well.
If you are using in-house cabling, move the computer into the room where the cable modem is, and use a known good jumper to try it, also try a different output port on whatever you are using to distribute (modem or router) different ethernet ports to the various computers.