Laptops running Windows XP have a feature allowing you to shut down the power for a particular USB controller (that may seems quite useful when the battery is low" , but it's still questionable). To disable this feature:
- Right click on My Computer> Properties > Hardware tab
- Click the Device Manager button.
- Double-click tthe USB controller (for all your USB devices)
- Open the Power Management tab in the window that appears.
- Then uncheck the following "Allow the computer to turn off this device to save energy".