usb port unknown device

i have a sony vaio laptop (xp pro) which has 2 usb ports but only 1 of which can be accessed when the laptop is docked and charging. everything worked great but then suddenly for some reason the front port stopped working. any usb device that i now plug into this port just comes up as an unknown device in device manager. if i use the other port they get recognised and the drivers get picked upas normal. there is power going to these devices through the problem port, but i cannot understand why it is suddenly playing up. any ideas welcome...