Wanna be extra careful? To prompt for a password each time Yahoo! Messenger is launched:

Click the Messenger menu in Yahoo! Messenger and select "Sign Out."

Click the "Yes" button.

Uncheck the option "Remember my ID & Password." This will cause Yahoo! Messenger to request a Yahoo! ID and password each time it is started.

Please Note: Yahoo! takes your security seriously and takes reasonable steps to protect your information. To learn more about these security steps, please read the Security Information in the Yahoo! Privacy Policy. Also, please visit our Security Center for additional information on how to reduce your security risk when online.