Adding this to your httpd.conf will make everyone log in except those coming from 127.0.0.1 (locahost). Watch out for sections which say "Allow from all" though, as that may override this.
<Directory />
Satisfy any
Order deny,allow
Deny from all
Allow from 127.0.0.1
AuthUserFile /path/to/htpassword
AuthName "Members Only"
AuthType Basic
Require valid-user
</Directory>
edit - This will protect vhosts as well, as long as they're under "/". I'm not sure how the directory is specified in windows.