To limit your application to a single instance on any given machine, add this loop to your application startup code. The loop iterates through the Processes list looking for an existing instance of your application. If it finds one, you can do whatever you like, such as alert the user that the application is already running, and abort the launch:
For Each item As System.Diagnostics.Process In _
"<your process name here>")
' Your process was found. Do something here, such as Application.Exit.