A common reason for such problems is the Power Management function of the USB root hub. Disabling the power management for all USB root hubs in your system should resolve this problem:

1. Open the Device Manager (Start-> right click on "My Computer", click on "Properties" -> Hardware -> Device Manager)
2. Look under the category Universal Serial Bus controllers and find the USB Root Hub.
3. Right click and select Properties, go to the Power Management tab
4. Uncheck the box "Allow the computer to turn off this device to save power."

Repeat this for each of the USB Root Hubs shown in the Device Manager.