With Vista approaching in the not too distant future, the links below describe a version of Windows XP Embedded SP2 designed to run on older PCs. It acts as a thin client allowing you to run a limited set of applications like the browser, RDP client, or Citrix client. Your existing management infrastructure like AD, Group Policy, etc. can be used to manage these machines. This provides an option for moving off any remaining Win 9x, NT, 2000 you may have on older hardware until your next hardware refresh.
"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. "
"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."