prevent multiple instances of an application

[Alister Pillow]

Hello and greetings from sunny Adelaide,

Does anyone know how to prevent launching multiple instances of a  
Standalone in Windows?

I have added a url protocol to the Registry that launches my app when  
a (custom) link is clicked in a web-browser. I would like the app to  
remain open and respond to further messages. Instead, each time a  
link is clicked, another copy of the app is opened.

When searching the web for an answer to this, all I could find is  
examples (in VB or Delphi etc) of an app checking to see if another  
copy of itself is running. Surely there must be an easier way?

Thanks,
Alister.