How do I download Yahoo Messenger for Vista?
Open your Web browser and navigate to the Yahoo homepage. Click the “Messenger” button on the left under “My Favorites.” Click the “Download Now” button, and then click it again on the next page. Run the installer. Check off any extras you want to install with the Messenger, and then click “Next”; you can download “Yahoo! Browser Plus” and make Yahoo your homepage. Accept the terms and agreement, and then click “Next.” Click the “Install” button on the last page of the installer and wait for the program to be installed on your computer. Click the “Finish” button and launch Yahoo Messenger right away.Source:Top Bits: How to Install Yahoo!