A default email client is not installed by Windows 7 by default. You either can download the Windows Live Essentials package which contains Windows Mail or go with the better choice, Mozilla Thunderbird.
Once you've installed either one, they will be registered as the default email client.
Windows mail can be retrieved from here -> http://download.live.com/wlmail
Mozilla Thunderbird can be retrieved from here -> http://www.mozillamessaging.com/en-US/thunderbird/
Considering this question pops up almost daily, I wonder if a posting explaining this should be stickied?