To invoke notepad.exe or any other process, you can use the Process.Start method defined under the namespace System.Diagnostics.
Note that you do not need to provide a full path for notepad.exe. As long as it is found in the system path, it would execute the process the successfully.
If you want to pass arguments to the process, then use the class ProcessStartInfo that can accept the program name and arguments.
ProcessStartInfo info = new ProcessStartInfo("notepad.Exe", "a.txt");