private void DataGridView1_MouseClick(object sender, MouseEventArgs e)
{
   DataGridView1.ClipboardCopyMode = DataGridViewClipboardCopyMode.EnableWithoutHeaderText;
   DataGridView1.SelectAll();
   Clipboard.SetDataObject(DataGridView1.GetClipboardContent());
}

using System.Diagnostics; // REQUIRED
private string programStartupDir = "";
private string logFile = "";
...
private void Form1_Load(object sender, EventArgs e)
{
   programStartupDir = Directory.GetCurrentDirectory();
   programStartupDir += "\\";
   logFile = programStartupDir + "Application_Log.txt";
}
...
private void menuItemShowLogFile_Click(object sender, EventArgs e)
{
   Process showFile = new Process();
   showFile.StartInfo.FileName = "notepad.exe";
   showFile.StartInfo.Arguments = logFile;
   showFile.Start();
}