All USB devices show as "Unknown" or failed

USB devices have all failed. When they start up now they show as unknown devices (all worked fine before - camera, printer, scanner, etc).

When any device is attached, XP (SP2) reports "USB device connected" then 10 seconds later says "device driver loaded" then a further 10secs later it says "USB device has malfunctioned or is unknown". Why?

All these devices have worked in the past. Nothing was added (hardware or software) to cause this change. I have;
1. Uninstalled and reinstalled ALL the affected hardware drivers
2. Ensured BIOS settings are correct
3. Rolled back to a different system point
4. Unloaded all USB drivers in Device Manager (to get them to reload on the next reboot)
5. Re-loaded XP (in desperation!)

all to no effect!!!!!!! ARRRGHHHH!!!!!!!!!!! WTF is going on?