Microsoft Knowledge Base Article 324915 contains the following summary:
You can use the Microsoft L2TP/IPSec VPN client to make connections to a Microsoft Windows 2000 or Windows 2000-compatible VPN server from a computer that runs any of the following operating systems:
|•||Windows 98 (all versions) with Microsoft Internet Explorer 5.01 or later and the Dial-up Networking version 1.4 upgrade.|
|•||Windows Millennium Edition with the virtual private networking (VPN) communications component and Microsoft Internet Explorer 5.5 or later.|
|•||Windows NT Workstation 4.0 Service Pack 6 (SP6) with Remote Access Service (RAS), the Point-to-Point Tunneling Protocol, and Microsoft Internet Explorer 5.01 or later.|
NOTE : L2TP/IPSec support is already built into Windows 2000 and Windows XP.