The PSU i'm using is an Enermax 400W - i've checked the voltages whilst the PSU is under load, and everything is well within tolerance - I've tried a 600W noname PSU to the same effect.
It takes place on all USB ports both front and rear - the USB devices that are problematic include a couple of small MP3 players, my Nokia phone, and a couple of external storage drives - there appears to be no rhythm or reason to what works and what doesn't - my ipod works fine.
BIOS: F7d - the latest for my board, i've tried older BIOSes also, and resetting the bios to defaults.
OS: WinXP
Hardware: AM2 processor, WD Harddisk and PCI Intel-NIC