I haven't messed with sftp servers on windows but I've read good things about core ftp. Check out the mini server option. Filezilla server does not support sftp(the client does though)
This is a free SSH server program. Very simple and easy to use. Just open it, put in a desired user/pass and directory and hit start. Instant Secure FTP server. Just forward port 22 on your router to this machine and you're good to go.