C’est LA nouvelle de cette fin avril et la fonctionnalité qui va certainement enthousiasmer le monde de l’entreprise. « Windows XP Mode » est spécialement conçu pour aider les petites entreprises à passer à Windows 7. Le « Windows XP Mode » vous offre la souplesse nécessaire pour exécuter de nombreuses applications conçu pour Windows XP sur un PC équipé de Windows 7.
Tout ce que vous avez à faire est d'installer vos applications sur un Windows XP virtualisé, via la technologie Virtual PC. Toutes les informations seront accessibles depuis le bureau Windows 7 et vous pourrez exécuter les applications directement à partir de Windows 7.

http://blogs.technet.com/win7fr/archive/2009/04/25/faire-tourner-des-applications-windows-xp-sur-windows-7-oui-c-est-possible.aspx

Quelques détails egalement ici :

Rafael Rivera and Paul Thurrott reveal a new Windows 7 application compatibility feature called Windows XP Mode. Yes, it's that "secret new feature" you've been hearing about ...

Over a month ago, we were briefed about a secret Microsoft technology that we were told would be announced alongside the Windows 7 Release Candidate (RC) and would ship in final form simultaneously with the final version of Windows 7. This technology, dubbed Windows XP Mode (XPM, formerly Virtual Windows XP or Virtual XP, VXP), dramatically changes the compatibility story for Windows 7 and, we believe, has serious implications for Windows development going forward. Here's what's happening.

XPM is built on the next generation Microsoft Virtual PC 7 product line, which requires processor-based virtualization support (Intel and AMD) to be present and enabled on the underlying PC, much like Hyper-V, Microsoft's server-side virtualization platform. However, XPM is not Hyper-V for the client. It is instead a host-based virtualization solution like Virtual PC; the hardware assistance requirement suggests this will be the logical conclusion of this product line from a technological standpoint. That is, we fully expect future client versions of Windows to include a Hyper-V-based hypervisor.

http://community.winsupersite.com/blogs/paul/archive/2009/04/24/secret-no-more-revealing-virtual-windows-xp-for-windows-7.aspx

http://community.winsupersite.com/blogs/paul/archive/2009/04/25/windows-xp-mode-internals-part-1-overview.aspx

http://blogs.technet.com/fabricem_blogs/archive/2009/04/27/xp-mode-pour-windows-7-kesako.aspx