After you have fixed the problem with scanstate.exe can't access down level manifests, (http://blogs.msdn.com/alex_semi/archive/2009/12/10/when-upgrading-or-reinstalling-windows-xp-to-windows-7-using-usmt-4-0-to-migrate-system-and-user-state-settings-like-wallpaper-and-network-printer-mappings-are-not-migrated.aspx)
you may found that now it's migrating too much - for instance, the "windows classic" theme is getting migrated from XP and your migrated customers are getting non-aero look of their Windows 7.
To tell it not to migrate the theme, you should use config.xml file. First, create one by running scanstate.exe /genconfig:yourconfig.xml on your source XP machine. Find the "theme" keyword and set migrate="no"
Then, copy captured file to your x86 USMT4 package folder.
So, now, if you use MDT extensions, you should see it under C:\_SMSTaskSequence\USMT\config.xml
Then, you need to set OSDMigrateAdditionalCaptureOptions to /config:"C:\_SMSTaskSequence\USMT\config.xml" in "set variable" step in your Task Sequence right after "Capture Network Settings".