I assume that you're using Gmail via its website. (You can also configure email clients, such as Outlook Express, to work with Gmail).
Then I get confused! Clicking on an email address in a webpage is
meant to open your default email client (such as Outlook Express) so, if you're actually clicking on the email address of a person you want to reply to, I'm unsurprised that it's happening. You should be using the 'Reply' option, as described here:
http://support.google.com/mail/bin/answer.py?hl=en&answer=6585
So what is it that you're actually clicking on?