[KB] Unpredictable behavior if you migrate a roaming user profile from Windows 8 to Windows 7
Windows 7 and Windows 8 have different user profile formats. When a user who has a Windows 7 profile signs in to a Windows 8-based computer for the first time, the user profile is updated to the new Windows 8 format. After this occurs, the user profile is no longer compatible with Windows 7-based computers. See the "More Information" section for details about how this issue affects roaming and mandatory profiles.
http://support.microsoft.com/kb/2748329/es-es?sd=rss&spid=16799