Return to an already open application when a user tries to open a new instance
I found this code to be useful. It does the detection and optional activation of an existing application:
If you're using .NET, this seems easier and more straightforward using build-in .NET functionality:
The Weekly Source Code 31- Single Instance WinForms and Microsoft.VisualBasic.dll
These link may be of help:
http://www.ai.uga.edu/mc/SingleInstance.html
It has code to detect another instance running, not sure what you can do with it once you've got the instance though.