Security against stolen desktop machines

I an interested in ways to protect the desktop from being stolen. We have flat screen monitors and the Dell DHP cpu’s, these are both easily fit into backpacks or winter coats. Is there anyone who has suggestions on security? Is there a way to notify a server if the monitor is disconnected from the PC? This is a call center environment, so PCs arent assigned to just one user, and assigned seating isnt an option either.