To make Yahoo webmail your default E-Mail:
CLICK HERE to install the Yahoo Toolbar.

In Internet Explorer, click Tools -> Internet Options -> Programs -> Email -> Yahoo! Mail -> OK
In Mozilla Firefox, click Tools -> Options -> mailto -> Use Yahoo! Mail -> OK

(optional) Change the E-Mail shortcut at the top of the Start Menu:
In Windows XP/Vista, right-click Start -> Properties -> Start Menu -> Start menu -> Customize -> Email -> Yahoo! Mail -> OK -> Cancel
In Windows 7, create a new shortcut to ["C:\Program Files\Internet Explorer\iexplore.exe"] or ["C:\Program Files\Mozilla Firefox\firefox.exe"], then right-click the shortcut and "Pin to Start Menu"
(In Windows 7 64-bit, it will be "Program Files (x86)")