I installed LAN messenger in my office machine from here. Can my network admin monitor the private chat or file transfer activities between me and my colleague? If the admin can in fact monitor the activity, then can anyone suggest any other messenger that is more secure?
The Features page on the website you provided says:
So you should be fine
The Short Answer would be yes, Some remote monitoring tools will able to track down remote users monitor screen where it prevents the unwanted usage of office system turning to personal system.
For e.g tools like activity monitor
In market there were many remote spy tools are available which makes the access to admin for looking/spying the monitor of client machine .