For some purposes you may find you want to log everything that appears on your screen. You can do this using the ‘Logging’ panel in the configuration box.
To begin a session log, select ‘Change Settings’ from the system menu and go to the Logging panel. Enter a log file name, and select a logging mode. (You can log all session output including the terminal control sequences, or you can just log the printable text. It depends what you want the log for.) Click ‘Apply’ and your log will be started. Later on, you can go back to the Logging panel and select ‘Logging turned off completely’ to stop logging; then PuTTY will close the log file and you can safely read it.
See section 4.2 for more details and options.