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".