Shared Sub Main()
' check for previous instance
If PrevInstance() Then
MsgBox("Application is already running!", MsgBoxStyle.OKOnly Or MsgBoxStyle.Critical)
Else
' Runs the application.
Application.Run(New YOURMAINFORM)
End If
End Sub


Private Shared Function PrevInstance() As Boolean
' array of running processes with this assembly name
Dim p() As Process = Nothing

Try
' put the processes into an array
p = Diagnostics.Process.GetProcessesByName(System.Reflection.Assembly.GetExecutingAssembly.GetName.Name())
' if there's more than one process with this name
If p.Length > 1 Then
' program is running...return True
PrevInstance = True
Else
PrevInstance = False
End If

Catch ex As System.Exception
' set return to False
PrevInstance = False
Finally
' clean up
p = Nothing
End Try
End Function