The following upgrade paths are supported by Microsoft:

Microsoft Windows 3.x

No supported upgrade paths

Microsoft Windows 95

No supported upgrade paths

Microsoft Windows 98\Microsoft Windows 98 Second Edition\Microsoft Windows Millennium Edition

Windows XP Home Edition Retail (Full) Version
Windows XP Home Edition Upgrade Version
Windows XP Professional Retail (Full) Version
Windows XP Professional Upgrade Version

Microsoft Windows NT 3.51 Workstation

No supported upgrade paths

Microsoft Windows NT 4.0 Workstation

Windows XP Professional Retail (Full) Version
Windows XP Professional Upgrade Version

NOTE: Windows NT 4.0 requires SP5 or greater prior to the upgrade.

Microsoft Windows 2000 Professional

Windows XP Professional Retail (Full) Version
Windows XP Professional Upgrade Version

Microsoft Windows NT 3.51 Server

No supported upgrade paths

Microsoft Windows NT 3.51 Server with Citrix

No supported upgrade paths

Microsoft BackOffice Small Business Server

No supported upgrade paths

Microsoft Windows XP Home Edition

Windows XP Professional Retail (Full) Version
Windows XP Professional Upgrade Version