Here's a great utility if you run a home network. As for what it does, the blurb on the home page says it better than I ever could, so I might as well just paste it for you:
"ShareWatch allows you to see who is connected to a computer and what files they are accessing. Have you ever wondered why your hard drive or modem is active, but you aren't doing anything to cause it to be active? It is possible a remote user is accessing your computer. Have you ever had a file locked but you don't know why? ShareWatch can tell you if a network user is using the file and allows you to disconnect them so that you can edit/delete the file. Have you ever wanted to shutdown a computer, but don't want to drop people using the computer. ShareWatch will show you all resources in use by remote users."
Simple, eh? But it works brilliantly on all recent versions of Windows, is totally free, and is just a 33 KB download. What more can I say? As you can see from the screen shot below, its display is very easy to understand. Here, I can see that someone in the lounge is logged into my office PC in order to play some music.
If this sounds like the sort of tool your need, get it from www.stevemiller.net/sharewatch/.