Seriously, SSH and WinSCP is probably your best bet. If you have SSH, you've got SFTP, which is like FTP, only "S"ecure!

That being said, the N800 really isn't a sensible file server. It is a mobile device, and having an external device dangling off of it makes it not-very-mobile. It makes an excellent client for a fileserver, however! Get the free MiniSFTP server from Core, install it on your Windows box, and then install sshfs or Wizard Mounter on your N800, and you can access the files from anywhere you can get Internet.