Windows XP Mode
An add-on capability available from Microsoft for the Windows 7 operating system. Windows XP Mode is designed to provide Windows XP application compatibility for Windows 7. It provides a virtualized environment to run Windows XP applications that won't run directly on Windows 7 itself.
See "XP Mode Ready by Windows 7 Consumer Rollout" on InternetNews.com
See also "Windows 7 Preview: Why I Like Windows 7" in Webopedia's Did You Know...? section.