Since they did not include Outlook Express in Win 7, you must install your own email client. There are several good ones.
Microsoft now uses an improved(?) version of Outlook that comes in a package of programs called Windows Live Essentials. Go here to download and install it.
http://windows.microsoft.com/en-US/windows-live/essentials-other-programs

I personally use Firefox's Thunderbird, and I am quite happy with it.