Windows will pop up the default email program when you click on a
'MailT0' link on a web page.
It would help if you disclosed which email service you *do* use.
That way, we might be able to suggest how to set that as the default,
which will keep Windows Mail from popping up.
Both Yahoo and Gmail claim they have a procedure for setting their
webmail service as your default email, but those procedures were
developed for Windows XP, and I'm not so sure those will work for
Vista. It doesn't hurt to give them a try: