BUFFY is probably the world's simplest FTP server for Windows. It is available at: http://www.smotricz.com/opensource/buffy/ .
BUFFY is an acronym: It stands for "Basic Universal Filetransfer For You". OK, it's a bit of a contrived acronym. As evidenced by the program's home page and splash screen, the author seems to have a crush on the main character of Buffy the Vampire Slayer.
An industrial strength, full-featured FTP server will have all or a large subset of the following features:
BUFFY has none of these features. Outside access is controlled by a single, case insensitive user ID and password, both of which default to "BUFFY" at installation. The default file directory is the directory BUFFY was started in, but it will CD to pretty much any other directory. Its default and only file transfer mode, both upstream and down, is ASCII.
After starting, BUFFY minimizes to a tray icon. Right-clicking on the tray icon gives access to a tiny GUI which allows changing the user ID, password and default (starting) directory. Thanks to a bug, changes from the GUI don't take effect until the next restart of BUFFY. The tray icon also gives access to a HELP|ABOUT dialog and the ability to EXIT.
BUFFY is a little less secure than a wide open screen door with a red carpet leading up to it. The author makes no bones about this. Because the user ID, password and data are transmitted in the clear, FTP is an easy protocol to snoop, intercept, spoof or what-have-you. This is well known in the networking/admin world, so most installations now eschew FTP in favor of SFTP and other secure file transfer protocols. BUFFY extends FTP's concept of insecurity in a consistent manner. Snooping or other attacks are unnecessary: A casual visitor to the PC running BUFFY can bring up the GUI to view both the user ID and password. Alternately, he can view its initialization file, BUFFY.INI, which contains the user ID and password in cleartext form. In view of its blatantly nonexistent security model, BUFFY is intended to be used as a quick-and-dirty file server within otherwise secure or trusted networks, or in cases where security is simply not an issue. The author assumes no responsibility for damage resulting from intrusions exploiting BUFFY's many security flaws.
Installation is a simple matter of copying the .EXE file to the directory it should run in. There is no installer. Creating a shortcut is optional and an exercise left to the user.
BUFFY seems to work well for ASCII file transfers to and from a Windows machine. It runs on flavors of Windows from Windows 95 upward. As found on the site, it is not ridden with trojans, viruses or other malware. Source code (in Delphi) is available on the site. | 
