No one is getting into your system. This only means you have File Sharing turned on and you can see another computer within range. It probably is a neighbor. If it bothers you can turn off file sharing (System Preferences - Sharing - de-select File Sharing) or you can make it so that Shared items are hidden in Finder. Open a Finder window move your cursor over the Shared items on the left pane then click hide.
thanks for your suggestion. However, I have 3 iMac's and 1 MBP in the office. They all have File Sharing turned off. They only have Screen Sharing turned on. And all of the Mac can see the unknown PC.
I had never had such problem in Snow Leopard before. Do you think it might be Lion's new feature AirDrop so that I can see other peoples PC despite they are in different workgroup? But if so, I should see all my neighbor office PC....but I don't see them excpet 1 PC.
This has nothing to do with Air Drop, that only works on Macs and even then not all Mac. Here is Apple's information about Air Drop compatiblity.
Again your systems are secure and they can only see there is a PC out there however if file sharing is turned off then you have nothing to worry about. Again if you want to not be able to see the PC in a Finder Window hover the mouse over the left pane where it says "Shared" then click hide and it will no longer be seen. Another (and easier method) is in Finder - Preferences - Sidebar tab - deselect what you don't want seen under Shared.
Yet to mention, I have 4 public PC sitting in the office and their names are showing up correctly and as expected. It is just the unknown PC showing up on the sidebar. I am sure I don't have that PC in my office as all my PC are enumerated in alphabetical order.
I prefer not to overlook the unknown PC by just hiding the share menu. I blieve if things are setup correctly, the name should show up correctly.