If you are using Windows 7-8....
Go into Device Manager and drop down Universal Serial Bus Controllers.
Right click one of the hubs in here and and select Properties.
In here there will be a Power Management tab, where you can select to allow the computer to turn this device Off.
See if that helps.
You can also check your BIOS to see there might be a "Wake On" switch set to USB... if so, disable it.