This happens because you
have hidden directories under the directory you are trying to delete.
FrontPage Server extensions create the next tree of system or hidden
directories under each web or sub-web root:
/_vti_bin
/_vti_cnf
/_vti_log
/_vti_pvt
/_vti_txt
/images/_vti_cnf
/_fpclass/_vti_cnf
/_private/_vti_cnf
/_vti_bin/_vti_adm
/_vti_bin/_vti_aut
/_vti_pvt/_vti_cnf
Plus, there is a hidden _vti_cnf directory in every content
folder.
Never use FTP to delete content published with FrontPage.
When you remove a sub-web using FrontPage client, not all the
files can be deleted right away because some Front Page ISAPI
DLLs are loaded by the web server into memory. These directories
will be eventually deleted by the system when IIS releases the
lock.
In the case you deleted several FrontPage folders using FTP and
your FrontPage web is not accessible anymore, please ask support
execs to repair frontpage extns on your website