Network Security Map Adapter Manufacturer

In the Network Security Map, there is an "Adapter Manufacturer" field for each device. Where does this information come from? Of all my devices, the only one to read "Unknown" is a PC running Norton Internet Security 2009.

 

The rest, including games devices, NAS devices and music players all contain the correct information in this field. Therefore, I thought it was a bit odd that my PC has "Unknown" there.