FTP server in IIS

I’m running IIS 4.0. I set up ftp server with anonymous, which I don’t give write permmision when creating a virtual site. Now I want to setup a site that allow permission on specific users to log on. I tried to give read/write permission, but seem like anyone can get in that folder and delete the information. I check the NT permision on the folder that only domain user has write to enter. What should