The easiest way is to download from dell's website.
You'll need your model number. It's easier to use the express service tag to search as different configurations may be available.
If your desktop has been modified before or if dell gives you a few choices, open the adapter properties in device manager, click on details and hardware id. Take the 4 character vendor (vid) and part (pid) ids and Google them to find out which nic model you have.