The sockets at the front of the machine are fed off contacts on the motherboard. These are sometimes individual blocks at the end of each wire rather that a single block plug, an, if any one of these is loose or one the wrong contact, the socket will not work. So its worth checking these first. Also, do the sockets on the back of the PC work? If not then I would suspect faulty USB drivers rather than hardware problems. If those on the back work, then those on a PCI card will also work.