According to Microsoft "Outlook Express isn't available in Windows Vista or Windows 7—you can't download it or run it on either version of Windows. However, you can install an e‑mail program from another company or use Windows Live Mail—a free program that is part of Windows Live Essentials."
However if you have Windows 7 Professional you might be able to run it in one of two ways:
1) Use "XP Mode"
2) Use Windows XP in a virtual operating system environment using Microsoft Virtual PC 2007
Source: How to use OE in Windows 7 - http://www.oehelp.com/oenwin7.aspx