As with previous versions of Windows, Windows Vista lets you easily choose between various email applications (Windows Mail, Windows Live Mail, Microsoft Outlook, etc.) to pick the default mail handler. (To find this: Right-click the Start button/orb, choose Properties, click Customize next to Start Menu and then view the E-mail link drop down at the bottom of the dialog.) What it doesn't let you do is choose a Web-based mail service like Gmail or Yahoo! Mail.
Fortunately, you can work around this courtesy of two free tools:
gAttach is a free program that integrates Gmail into your system as your default email service, to be used by default instead of a local client when you send email or attach files from the desktop, the browser, or any local applications.
yAttach! allows you to easily attach files to new messages in the all-new Yahoo! Mail. All the features available in Windows that were once only available to your desktop mail-using friends are now available to all Yahoo! Mail users.