PC as in "a box equipped with Windows crippleware" I assume
With any real OS, you'd just use the builtin SSH/SFTP functionality - dead simple and secure.
With Windows, it kind of depends on what exactly is the setup and the purpose.
- If you don't mind slow transfer speeds and you don't want to mess around with firewalls etc, then you could use almost any IM service. Unsuitable if there are many files.
- Use the DCC feature or IRC. Unsuitable if there are many files.
- Set up some userfriendly FTP server program on one of the PCs.
- Set up a web server and some upload scriptie thingie. That's just silly unless you aleady have a running webserver.
All of the above are insecure to some extent..
- Use the file transfer feature of some remote control pograms like Radmin, PCanywhere or VNC. May be insecure depending on the program.
- Set up a SSH server in one of the PCs. Either tricky or expensive when using Windows as the server. Pretty secure.
In addition to the above direct PC to PC transfers you can make use of dozens of schemes using an intermediate server..