I think the latter. I re-installed it with the OS disk that came with my computer.

How did you reinstall Windows and what version? If it's from the Dell CD it should have installed anything that came installed on your PC. If it was with a Windows CD then you may have to install drivers for some of your hardware with the Dell CD or the OEM CD/floppy.

After installing the Operating System you must install the chipset drivers before any device drivers.  The chipset drivers define what devices are on the motherboard or connected to it.

 

The install sequence is (1) Install Operating System (2) Install chipset drives (3) install device drivers (4) reinstall applications and backed up user data (5) reinstall peripherals.

what if I don't know what any of those things are?

The chipset drivers are for the motherboard and define all the devices.  If you don't have them or can't find them you can download them for your specific model from the download site. 

 

Run the install and then restart Windows and see if the network (ethernet) is detected.