Questionable Searches in the Workplace

We recently has an issue at my workplace about some searches and visits to adult websites that were showing up in the history on a computer that approximately three people accessed during the day. It was a touchy situation because two of the three people were women who with out question were not viewing this particular subject matter. Finally the problem escalated forcing our boss to address the issue with the third employee who denied that this was his doing and insisted it had to be the cleaning lady who was responsible. This created a very difficult work environment for some time as everyone was suspicious of each other. Had our business installed keystroke logger software on the computers when the problem first occurred it would have provided direct evidence as to who was conducting what searches and likely would have dissuaded any future searches for inappropriate material.