How to Set Up Yahoo Mail as a Default Email Client in Windows 7by Andrew Meer
Yahoo! Mail is an email client that you can use to send and receive email messages. The default email client on your computer opens up automatically when you click on an email address on a website or a document. If you want to set Yahoo! Mail as your default email client, you must first install the Yahoo! Toolbar and then configure your computer using the Default Programs application.
Open your Web browser (Internet Explorer or Mozilla Firefox) and go to the Yahoo! Toolbar webpage (toolbar.yahoo.com).
Click the "Download Now" link.
Check the box next to "Install Mail Plug-in and make Yahoo! Mail the default Mail Provider."
Click "I Agree" and then click "Install." Follow any other onscreen instructions to install the Yahoo! Toolbar.
Click the "Start" button on your desktop, type "Set your default programs" (without quotes) and press "Enter."
Click "Yahoo! Mail" and then click "Set this program as default" to set Yahoo! Mail as your default email client.