>I got an application which can be passed a parameter to view on startup. This is find, well and good until I wanted to have only one instance of the app running at any one time. If the app is already running, how can I pass it the new parameter to load?

You need an IPC mechanism; in addition to using DDE as Sergey mentioned, one of the best mechanisms available is to create an out-of-process COM server as a part of your application - you would then simply set and get properties and invoke methods of the COM server running as a part of your app from any COM-capable application.