My son just re-installed Windows XP, but for some reason Windows isn't recognizing that there is a network card. In Device Manager, there is no entry for the item, even though an inspection of the motherboard shows that there is an ethernet port, and its LED is lit with the cable plugged into an active broadband connection.
This is an on-motherboard networking "card", btw. Unlikely to go missing.
The thing was working previously, there was no hardware problem at the time of the re=install.
Edited to Add: This turns out to be an IBM ThinkCentre PC.