FrontPage only lets you password-protect an entire web or subweb (a subweb being a FrontPage web created within another FrontPage web). Select Tools | Security | Permissions (see Figure 1). Tell the Permissions dialog to use unique permissions for this web, click Apply, and then click the Users tab. Select the radio button Only registered users have browse access and then OK to change the permissions. You can then call up the Permissions dialog again to add users and their passwords, then e-mail them and supply them with the URL and username/password combination.

You cannot password-protect a single file from within FrontPage itself, but you can work around this limitation and password-protect a single page by creating a one-page subweb, setting its permissions, and then linking to that page from the parent (or other) web. Actually, this isn't a bad idea anyway; you should keep the interactive elements of your site separate from the rest.

<<shamelessly borrowed from : http://www.zdnet.co.uk/help/tips/story/0,2802,e7107400,00.html>

hope this helps.

cheers,
nick