Gary Kessler, 52, is an associate professor of computer science at Champlain College in Burlington, Vt. Mr. Kessler has worked as a computer-forensics consultant on the side for the past five years. Computer forensics is the process of gathering evidence from a computer. “Frequently, you are looking at files that someone thinks are deleted,” Mr. Kessler says. “It turns out that when you use a computer almost everything you do leaves traces, and it is next to impossible to delete information from a computer…”