Gmail isn't a program, but something like an email provider or a website. So that makes it somewhat difficult.
But if you couple your gmail-account via IMAP to an email client (like Thunderbird), you can set that email client as the default program for mail on your PC. http://email.about.com/od/accessinggmail/f/Gmail_IMAP_Settings.htm tells how. It's not ideal, maybe, but it might work for what you want with it (sending a mail if you click on a mailto-link in your browser).
...email handler in Windows 7?