Filezilla Client also works really well as an FTP/SFTP client on Windows.

Filezilla can use PuTTY pageant for public/private key authentication via ssh.

If you follow my brief tutorial explaining how to configure ssh public/private key authentication when using PuTTY, you can then easily configure Filezilla SFTP in under a minute. The only requirement on the 770/N800 is openssh - no need for a seperate tftp/ftp server.