These are Frontpage files that should not be removed. They are the server side extensions that need to be installed if you want to use Frontpage to publish with and use any of the Frontpage components. This extensions enable the components to run server side; and can be easily broken.
_borders
_derived
_fpclass
_overlay
_private
_sharedtemplates
_themes
_vti_bin
_vti_cnf
_vti_inf.html
_vti_log
_vti_pvt
_vti_txt