I don't know if this is still current, but there was an issue with insufficient power being available at the USB port. The fix was to buy a powered hub.
Although all USB ports are limited to 500ma, whether your machine can support that, I suspect, will be determined by the size of the power supply you have, and how many peripheral devices fitted.
What happenes if you connect the modem after you have booted up? Remember, you can "hot plug" USB devices. Also, in Device Manager, are you showing a problem?