virtualbox.org

host: XP Pro SP3
guest: XP Pro SP3

VB default selected NIC: PC Net Fast III, Host only adapter

When I changed to: Intel Pro/1000 MT Desktop, NAT

after starting guest XP it naturally noticed the new hardware, and prompted for a driver. Other than scouring the interweb is there a place in VB directory structure where a driver might live? Or should automystically search itself -- as there appears no way to add alternative NICs to the VB machine manager

Why did you change it? PC Net is selected as the default because XP has a driver for it.

Depending on which version of VBox they have there was an issue with the PCnet driver in a few version back and suggestions were to use the Intel Pro/1000 MT SERVER because Windows also should have that driver. Note the SERVER at the end.

But if you have version 3.0.6 or newer you should not need to change the default.

In any case if you change or need/want to change it is like any other network issue. You need to have the drivers in the guest. VBox does not provide these the guest does.
You can switch back and the download the driver, switch again and install the drivers.