Articles: Windows Rootkits in 2005 - Part I and II
Both articles were published by Security Focus and they provide excellent technical information on how this emerging threat works in detail.
Windows Rootkits in 2005 - Part I
http://www.securityfocus.com/infocus/1850
Windows Rootkits in 2005 - Part II
http://online.securityfocus.com/infocus/1851