This should help: http://support.microsoft.com/default.aspx?scid=kb;en-us;305548&sd=tech.
Be sure to run Windows Update afterwards, because there may be some new patches that you need to apply to FTP. The security implications of installing FTP are pretty serious, and securing FTP can't be summed up in a few sentences. I would run the Microsoft Baseline Security Analyzer (MBSA) after you configure it, though. You can download MBSA here: http://www.microsoft.com/technet/security/tools/mbsahome.mspx.
This was first published in April 2004