Change the text in the email users get when they sign up to wp?

Is it possible to change the email users get when they sign up to WordPress?

As it is, they get an email telling them their username/password and the url to login, such as:
http://theisleofavalon.com/wp-login.php

The way I have my site set up, logging in and registering is all done within the main site, so could I just change that URL to point to:
http://theisleofavalon.com

The same goes for when they create a new site in Multisite, where it has the login link, can I just have this changed to point to the actual site?

Thanks.