The following adm should do the trick, but it is a preference and not a true
policy. That is to say, if you decide to delete this policy at a later date
the settings it makes will stay in the registry.
I have not tested this other than to make sure it does actually change the
registry in the right place, I leave proper testing up to you :-)
CLASS USER
CATEGORY "Custom Settings"
KEYNAME "Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders"
POLICY "Startup folder redirection"
EXPLAIN "insert your own policy explanation here"
PART "Location of startup folder" EDITTEXT REQUIRED
VALUENAME "Startup"
END PART
END POLICY
END CATEGORY