Dead-simple FTP server for Windows?
This one is definitely portable:
http://www.pablosoftwaresolutions.com/html/baby_ftp_server.html
Here is another option that is not portable:
http://filezilla-project.org/
Have you evaluated the native FTP Server feature?
I doubt it will perform as simply as you would like it. But maybe you weren't even aware of this feature.
Possibly relevant: How do I setup an FTP server on Windows 7?
This might also meet all your needs.
FTPDMIN: http://www.sentex.net/~mwandel/ftpdmin/