Follow frogjerk's advice, and check the BIOS for the NIC to be enabled
Listen to BurbankKarl's address as well. If the NIC is enabled in BIOS then the specific drivers for the motherboard are not being detected by Windows, it will not detect built in stuff correctly (Like modems, correct video card, NICs, correct soundcard, etc...). You would need to load the correct and updated drivers from DELL for the motherboard and onboard chipset.
Apparently I need to go to Dell.com on a different machine, plug in service tag #, etc., as some here have kindly advised, so that is what I will try next.