Hmm, well… you may have scripting disabled in your Internet Explorer.

To enable scripting:

Click the "Tools" menu on your Internet Explorer toolbar and select "Internet Options."

Click the "Security" tab.

Click the Internet (globe) icon and click the "Custom Level" button.

Scroll down to "Scripting."

Enable the following:

• Active scripting
• Allow paste operations via script
• Scripting of Java applets are all enabled

Click the OK button at the bottom of the window.

Note: Restart Yahoo! Messenger and you should start seeing text in your IM windows again.