Do you use Outlook at all? If not then uninstall it. (You can't "disable"
it)
Control Panel-Programs and Features, click on Office, click on Change then
select Add or Remove Features and make Outlook unavailable.
And BTW it's NOT "Vista" Office, it's Microsoft Office. Vista is an
Operating system.
A mailto: link loads whatever is currently configured as your e-mail
*client*, not some account defined within it. Yahoo Mail is an e-mail
service. You don't open an e-mail account. You connect to it. If you want
to use Yahoo Mail as your e-mail service then define a Yahoo Mail account
inside of Outlook. If you don't want to use Outlook as your default e-mail
client, choose another in Internet Options -> Programs.