The ways to transfer files between Linuses are clearly using sshfs sftp:// urls. Not much more to say on that, others did it well. For the file transfer between Windows and Linux, there are a couple of ways. The first is to use sftp, once it is installed on your server. You will likely have to install the package openssh-server. Search for this using Synaptics.
Then, under Windows, you can use filezilla to read and write files from your Linux server. It is strongly recommended that the actions be initiated from the Windows box. Installing and sharing files FROM Windows is not very stable, in my opinion. FileZilla will use sftp to transfer files.
The other way is to install samba and actually map a drive from Windows to it. You need to install the required samba packages on your server. You then also need to define a path under Linux and a sharename. Search for samba and Windows on the ubuntu web site. There are many web pages about this. Once the samba share is created, you can then map a drive to this share or use the "network neighborhood" to access them. One of the settings in the samba config file is the name of the workgroup. This name must match the actual name of the workgroup of your Windows machine.
Unless this is a connection that will be used on an ongoing basis, I strongly recommend that you then disable the NFS connection once you are done.