From Microsoft:
Microsoft Windows Fundamentals for Legacy PCs is a Windows-based operating system designed for customers who have older computers running earlier operating systems and who are not in a position to purchase new hardware. This operating system is available only to Microsoft Software Assurance customers and helps you get the most from your older hardware, thereby reducing your total cost of ownership. This Software Assurance benefit provides improvements to security and manageability to reduce the strain on your IT team, improve end-user productivity, and close security gaps.
Windows Fundamentals for Legacy PCs is based on Microsoft Windows XP Embedded Service Pack 2 (SP2), enabling you to upgrade to the security and stability of the Microsoft Windows XP platform. This eases the transition to Windows Vista Enterprise when you eventually replace your computers.
Windows Fundamentals for Legacy PCs is not a general-purpose operating system. It is designed to work with the Microsoft Remote Desktop Connection client or third-party clients such as the Citrix ICA client. In addition, it allows for a limited number of workloads to be executed locally, including security software, management software, terminal emulation software, document viewers, and the .NET Framework.
無恃其不來, 恃吾有以待也; 無恃其不攻, 恃吾有所不可攻也.