When debugging a program in Net Express is there a functionality which allows you to stop running the debugger when the contents of a data item is changed?

Yes, this is called a Break on Data Change and is available for data items displayed in the Watch List.  There is a STOP icon at the right of the Watch List control that enables/disables Break on Data Change. 

If you "Examine" a data item it is diplayed in an Examine List dialog box and you can "Add to List" to add it to the Watch List and from there you can toggle the Break on Data Change for the data item.