Unfortunatly Microsoft have removed Windows Mail from Windows 7 because of all the legal anti-trust stuff against them. You can read more about this here...http://news.cnet.com/8301-13860_3-10048142-56.html
Your options with Win7 are using Live Mail (http://download.live.com/wlmail)
or loading on Office and using OUTLOOK or use a download site like filehippo and get a copy of Thunderbird or another freebie email program.
If you want to use the original Windows Mail program (as with Vista), and you still have a PC with Vista installed on it, you can copy the program from one PC to another.
Here is a link that explains how to do this....www.sevenforums.com/tutorials/5481-windows-mail.html
Hope this helps ?