Hi ReiCardiz - You have realized that there is no default email client with Windows 7 (past versions of Windows included Windows Mail). You now have the option to download the e-mail client that you wish to use as default.
As Davcon suggested, Windows Live Mail is available as a free download with Windows Live Essentials (you can choose to download whichever programs you'd like, without having to download all of them). You can learn more about it here http://explore.live.com/windows-live-mail
Hope this helps!
Microsoft Windows Outreach