Type System Restore in Start search box. Look at the restore points after having it "Show more restore points."
Do you see a restore point from right before you made the change? You can roll back your Win7 to before you made the mistake.
If there are no System Restore points for that period then tell us exactly how and where you turned off the USB warning.
Once you recover the USB function you disabled, recreate the USB error message you were getting and either type the exact text out for us or the Google, or make a screenshot of the warning using the SNipping Tool in Start Menu, attach to post using paper clip in reply box.