>I need to be able to fun a routine on on the network asking who on the network has a specific file open (say invoicefile)
>
>I need to know the user name and the login name of each machine using this file (ie the sort of information I would get with sys(0)

To get it interactively is easy;
Go to mycomputer>manage
Then connect to server machine (rightclick>connect to other computer + machine name)

When connected to server machine go to
SharedFolder>OpenFiles

Find your file in list of open files

Now from within app is more difficult. Perhaps someone else will jump in with solution for this;

HTH