Well don't?
Make a USB cable for your Arduino that has the 5V wire cut in it, so you do not try and feed PC current into a project that is already 5V powered.
5V is not exactly 5V, some 4.9V others 5.1V some 5.0V, the differences in two supplies connected together can cause problems.
Tom... 
PS, this is my 5V isolation cable, only the 5V wire is cut and bought out to terminals, that way you can still use it as a powered cable and/or fit an ammeter to measure currnet.
