defamation

[ed_flynn@msn.com]

i recently went to work for major insurance company, 3 weeks after i started i was asked to meet with 2 people from the human resources department. during this meeting i was informed that a fellow employee accused me of having a sexual relationship with my sales manager, this was a lie and i eventually found out that the person that accused me of having this perverted relationship ( i am a male and so is my sales manager)had an affair with the sales manager. i was totally unaware of this situation and was not told about it for several days after the meeting with human resources, i have requested a copy of my personal file and the investigative report that was conducted, i was given my personal file a month later and was told that they would not give me the investigative report. this employee has had complaints from other employees, and the company doesn't seem to want to do anything about it. do i have any course of action to make sure this doesn't happen again?
any advice would be appreciated...

ed flynn