Treat the person like he or she is doing you a favor by bringing complaints to you. At least you are better off absorbing the negative attention than the outside world. These messages may be disproportionately negative, but they are a great way to check the pulse of your customer base. If you can't meet their needs, do something to show that their concern was not wasted. The long-term investment in maintaining a customer is usually more valuable than one individual sale.
Computers can be a great help in resolving complaint.
Customer records can help you understand problems. A troubleshooting database can help employees resolve various situations when they occur. If the problem isn't solved them, you could either lose a customer or be stuck as a problem escalates. If something sounds fishy, prepare to investigate. Without revealing your doubts, get as much information as possible and follow up later.