Putty : Clear the Scrollback Buffer

Putty 's scrollback buffer is quite useful in displaying the results of previous commands or for copying a session to the clipboard for pasting in another program. 

If you want, you can clear the scrollback buffer before entering a particular remote command, so that when you select, copy, and paste the scrollback buffer, only the results of your recent remote command are stored in the buffer. Just right-click the Putty menu bar in your current session and choose "Clear Scrollback".

From : http://malektips.com/putty_0004.html