Over-drawing your USB hub?
One of the nifty things about the USB bus is that it provides power to the devices plugged into it, providing of course that the device does not draw too much power. Each USB port has a 500 mA power limit, and if you plug too much gear into the port by using a non-powered USB hub, it would be possible to draw too much power from the port, and not all of your devices will work.
You can view the power demands on any USB port in Windows XP from your Device Manager. Simply go to your Device Manager, expand "Universal Serial Bus controllers", and double-click on USB Root Hub. Go to the Power tab and there you will see a list of any devices plugged into that particular port and the power demands that it has. Just add up the power demands of each and you will know what you need.
The other option, of course, is to use a USB hub which has its own AC adapter.