|I'm sure it would work just fine.|
While I'm not a huge fan of USB network adapters, I've recently added one to my work PC as it only has two expansion slots and one was in use by the graphics card. I now have 3 in this unit, the builtin NIC, the PCI NIC and the USB NIC.
You should try enabling your onboard NIC before you buy anything.
As wanderer said, watch the entire bootup sequence. The very first part is called the POST (power on self test) and it's usually during this phase you can see a message that says something like" Press delete to enter setup" and then you immediately press the appropriate button.
Once you're in the BIOS (Setup) carefully look through everything and find the onboard NIC settings. If it says "disabled" beside it, that's your issue.
Instructions for maneuvering through the BIOS should be along the bottom of your screen, or on the right hand side, once you've gotten into the BIOS. Usually it's the arrow keys, escape key and the Enter Key.