Try switching off Power Management for ALL the USB Root Hubs in the Device Manager i.e. In Device Manager>USB root hub properties>Power management> untick the box that "allows the computer to turn off this device to save power"
If your system keeps resetting this option see this workaround:http://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/windows-7-usb-power-management-wont-allow-me-to-un/f1a339b7-1388-4790-9cf4-3e895f4fb3f7
I've found that windows has a habit of switching off USB ports to save power whether you're using the USB ports or not. This freezes the keyboard/ mouse/ system and causes much frustration!