workstations move

I have this idea but I dunno how to achieve it. I want to write an executable that runs and detect if the user has moved from one workstation A to B, if the user is on B now, do something to A. Just don't know how to achieve it. Is there an API or something like that?

You mean user logs off of A and then goes and logs on to B? And, while he is on B he does something to A or you want to do something to A?