Yet another USB question

I'm trying to understand the USB listing in device manager (WinME).

My PC has 4 USB2.0 ports arranged in two pairs. With nothing plugged in,
device manager shows (the numbers are for reference):

(1) Universal Serial Bus controllers
(2) USB 2.0 Root Hub Device
(3) Universal Serial Bus controllers
(4) NVIDIA USB 2.0 Enhanced Host Contoller
(5) Standard OpenHCI USB Host Contoller
(6) Standard OpenHCI USB Host Contoller
(7) USB Root Hub
(8) USB Root Hub

How do each of the items relate to the 4 ports?

Phileas