By default, GNS3 uses the telnet application built into Windows or Linux. But if using Windows, you may prefer to use
PuTTY. Both of these terminal emulators are free to download at one of the following Web sites:
PuTTY: http://www.chiark.greenend.org.uk/~sgtatham/putty/
Just download the terminal program you prefer and install it on your
computer. Inside GNS3, choose Preferences under the Edit menu. Choose
General in the left pane, and the change the Terminal command box to
read:
For PuTTY: c:Dputty.exe -telnet %h %p
The above commands should be adjusted to point to the location where
installed the program. Note that with PuTTY instead of the built-in telnet
client, your router name will appear in the title bar of the window.
Ref : GNS3 site

thanx