Off-topic? USB ports on Asrock K741GX

Hi
I couldn't find an Asrock group so I thought I'd ask in here.
I have 2 new K7S41GX boards from different sources both with the same
fault (?) Operating system is XP Pro Service Pack 2. Only 3 out of the
6 USB ports work - 1 of the 2 front ones and 2 of the 4 rear ones. The
dud rear ports are the upper left and lower right ones (looking at the
back of the case).
I've tried disconnecting the cable to the front USB ports in case
there was a wiring problem on the header cable, but no change.
Also tried removing all the USB devices in Device Manager and letting
Windows find them again.
According to the driver CD, the board doesn't need any special USB2
drivers installing if you have Win XP Service Pack 1 or later.
I tried 2 different devices (memory stick and digital camera) with no
luck.
Any ideas? Am I missing something obvious?
DM