This is a diagram of upgrade paths between the various Microsoft operating systems that you can purchase through TechSoup. TechSoup is an organization that sells nonprofit organizations software at low prices. Generally, people in an organization want XP Professional, because it connects to Windows Server. If you buy a cheap computer today, it comes with Vista Home Basic... and there's no cheap downgrade path to XP Pro. (Meaning organizations have to buy a system with Vista Business preinstalled.)

What's kind of funny is that there's an upgrade path from Windows 98 to Vista.