Your PC most likely came with 3-5 discs, depending on what extras you got. To reinstall the needed drivers, you will need to put in the disk other than the one with the operating system on it, titled something like Drivers, Utilities, or something to that nature.
The disks I have seen from dell usually come with many more drivers than you actually have on your system, that way they can ship them with the coresponding models. You can either access the administrave tools found in the All Programs menu, or Control Panel, and find the type of Ethernet device you have, then install the driver that matches it off of the disk. Or you can go to Dell.com, and input your service # which will tell you all the information about your PC as they shipped it.
Hope this helps,