This snippit explains some, if not most, problems with USB modems and current draw. This applies to all versions of windows that support USB.
If no devices work when you plug the devices into the root hub, verify that the power requirements of the bus are not being exceeded. USB devices can draw a maximum of 500 milliamps for each connection. If a device attempts to draw more power than this, the specification recommends that the computer should be able to disable that specific port until the computer power is cycled (this is called "suspending" the port)
VIA and SIS chipsets had problems with determining the power usage and cut off early. Later drivers "fudged" the problem by shifting the cutoff up. USB2 and USB2 drivers have better current control.