Are the pc’s being used?

We have an enterprise Windows installation of 1000+ clients, and it continues to grow. I know I have pc’s that are underutilized, but how do I measure? Logon times won’t really help – someone can logon first thing in the morning, read their mail, then let the pc sit idle for the rest of the day. I want to assure the honkin’ big pcs are assigned to the power users, and not the squeeky wheels. Thanks.