Are you using the motherboard USB sockets or a PCI card or hub?
If motherboard, try plugging into a different socket If you have a choice of the main "bunch" of connectors (where the audio, parallel/serial ports are) or ones further down that connect to a front panel or similar, choose the main bunch ones.
If separate card or hub, connect direct to the motherboard.
So if i plug it in the front, it sometimes works. If i plug it in my motherboard sockets at the back, it isn't recognised. If i plug it in my USB2 card socket, it's cool - though i now need more USB2 sockets!
If you are using a number of USB devices at the same time and they are powered by the computer they will share the max power available for all yr USB skts which MAY well cause the symptoms you describe.
If you say you need more USB skts, go for a USB Hub with it's own separate power supply (approx £10 for 4 way if you shop around)