Ian in Northampton,
I will be interested in the replies that you get as I have been trying to sort out a similar problem on my 6 year old desktop, running Windows XP Pro, SP3 for some time.
I have tried loading a USB driver update, removing all the USB devices in the Device Manager and letting Windows reload them, updating the Motherboard BIOS, unplugging the mains power for several hours and holding in the power button for a minute, etc. all to no avail.
In the end I purchased a 5 port (4 external & 1 internal) USB2 PCI card from PC World for £10.95 and fitted it in a spare PCI slot.
Not the most convenient or elegant solution, but now at least I can back up at a reasonable speed (about a fifth of the previous time) and I don't get the "This device can work faster" messages. I use the original machine USB ports when the extra speed isn't needed.