It could be that it gets put to sleep for no good reason (it happens), try the following:
Go to your Control Panel > System > Hardware tab > and click on the "Device Manager" button.
In the Device Manager window, find the item or items labeled "Universal Serial Bus controllers." Open it, and double-click the first item called "USB Root Hub."
In the USB Root Hub Properties dialog, click on the "Power Management" tab.
"Allow the computer to turn off this device to save power" is checked by default.
Uncheck this box, and click "OK."
Repeat for all "Universal Serial Bus controllers." items if there is more than one.