If you deploy an Enterprise CA on Windows Server 2012 you will have to disable a security setting to enable Windows XP clients to enroll for certificates.  See: http://social.technet.microsoft.com/wiki/contents/articles/6289.certification-authority-authentication-level-incompatible-with-windows-xp-en-us.aspx.  Please note the "Warning" in the article, before disabling this setting.

-Chris