If you want personal chat with a friend
you don't need to download any yahoo messenger
All you need is your friends IP address and Command Prompt.
Firstly, open Notepad and enter:

@echo off
:A
Cls
echo MESSENGER
set /p n=User:
set /p m=Message:
net send %n% %m%
Pause
Goto A

Now save this as "Messenger.bat". Open the .bat file and in Command
Prompt you should see:
MESSENGER
User:
After "User" type the IP address of the computer you want to contact.
After this, you should see this:
Message:
Now type in the message you wish to send.Before you press "Enter" it should look like this:
MESSENGER
User:27.196.391.193
Message: Hi
Now all you need to do is press "Enter", and start chatting!