Searching for Inapropriate foot prints

For work, I need to search a few computers for internet usage that is suspect of violating company policy, as well as shared jokes that are of an offending nature and could cause the company liability. I am thinking the two users knows enough to delete History and Cookies. So where else would I look to see whats going on? I also need to see email packets for shared tasteless jokes and attachments. I am not interested in reading anyones mail, just am hoping for a way to screen for offensive materials. How do I do this. I have a server and I am the admin and am the company owner, but have never had this situation to deal with before. I am good enough to do what I need to do to back up data and trouble shoot basic issues but not enough to dig deep as needed in this case. Can anyone give me file locations to look at or whatever? Any advice would be helpful as I need to put a stop to this behavior in the work place before I get litigation. Where do I go from here??