This is what I found about Activity Monitor : (sorry but the Adm dosen't allow me to post the link)
"Ever dreamed of monitoring software to track your network activity in real time?
Ever thought of how useful it would be to know what your employees or students do when they are supposed to work or study?
Ever lost money and/or confidential information, lacking control over your staff online activity?
Activity Monitor is the solution you needed!
This powerful employee monitoring software allows you to track any LAN, giving you the most detailed information on what, how and when your network users performed. Whether it is a library public network, university or commercial organization network – Activity Monitor offers efficient control.
Our employee monitor consists of server and client parts. Activity Monitor server can be installed on any computer in the entire LAN. Remote spy software (Agent) is a small client program that is installed on all computers on the network you want to monitor. It can be installed even remotely from the PC with Activity Monitor on it or via Active Directory Group Policy in a Windows domain.
Any computer in the network under control can be spied remotely. You may tune this employee monitoring software so that it will record activity on all networked computers. This information can be later used for the deeper analysis and advanced report generation.
Activity Monitor has proved its excellent efficiency in increasing the overall productivity of the establishments where it was used to track the local networks. This powerful tool from SoftActivity saves your money."
It seems that someone from outside would spy our computers using this software. If the PC is on but disconnected the message doesn't appear, and inside the PC Avg doesn't see any danger. Surfing the net I found that since 2009 some other people, not many, have had the same problem, but none of them write they solved it.