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

Imports System.IO
Dim LogStream As FileStream 'should be at a level with enough scope for your logging

'Start the logging to your file - probably to go in your frmMain.load
LogStream = New FileStream(System.IO.Path.Combine(System.Environment.CurrentDirectory, "MYFILE.log"), FileMode.OpenOrCreate)

Dim objTraceListener As TextWriterTraceListener = New TextWriterTraceListener(LogStream)

Trace.Listeners.Add(objTraceListener)

'Whenever you want to log a message use the vb.net 
trace.writeline("message") 
'and it will get written to the log file.

'Close the log file when your application ends
If Not LogStream Is Nothing Then
Trace.Flush()
LogStream.Close()
End If