IIS web server hacked..any tips?

["Francesco" <francesco () blackcoil com>]

I have a Windows 2003 Server running IIS 6, SQL Server 2000, MailEnable,
and ASP.NET 1.1.  WWW and FTP are enabled, but restricted by IP.  FTP is
additionally protected by authentication.

Yesterday someone managed to access the server and dump 8GB of DVD files
into a deeply nested folder in a backup directory, for sharing I
presume.  The payload folder was NOT within the available folders given
access to FTP users.  Someone was able to "see" the entire D drive and
figure out a hidden enough location at their whimsy.

I thought the server was fairly well locked down, but apparently not.
What is the usual method of intrusion for "warez" attacks like these?

Francesco