I have had my S4 for a couple of weeks now. Just after getting it I connected it to my desktop computer (Windows XP, not an Apple) via USB and selected MTP on the phone. At that time I connected to the phone and was able to see folders on the phone. Since that time the phone or the desk top will not connect via USB. I get nothing on the phone or desktop other than the phone saying it is charging. What is the problem??