windows 2000 professional hacked with Serv-U FTP Server

From: Tony (tony.wong@sbcglobal.net)
Date: 11/24/02 

From: "Tony" <tony.wong@sbcglobal.net>
Date: Sun, 24 Nov 2002 05:18:34 GMT

Someone hacked into my windows 2000 machine running sp2 and installed Serv-U
FTP server and was uploading movies files to this box. This box was running
sp2. Looking in the local users, a bunch of users were created and belong to
the local adminitrator group. File and print sharing, was enabled on this
box.

How did hacked get into the machine and installed Serv-U ftp server?

How do I prevent this from happening?