nabend...
M$ in der EULA:
Q. Can I downgrade my OEM version of Microsoft Windows XP Professional to Windows 98 (Second Edition)?
A. Yes. OEM Downgrade Rights for desktop operating systems apply to Windows XP Professional only. Please note, OEM downgrade versions of Windows XP Professional
are limited to
Microsoft Windows 2000 Professional, Microsoft Windows NT Workstation 4.0, and Microsoft Windows 98 (Second Edition).
End users can use the following media for their downgrade: Volume Licensing media (provided the end user has a Volume Licensing agreement), Retail (FPP), or System Builder hologram CD (provided the software is acquired in accordance with the Microsoft OEM System Builder License). The downgrade operating system is governed by the Windows XP Professional EULA, and the end user cannot use both the downgrade operating system and Windows XP Professional. However, there are no downgrade rights granted for Microsoft Windows XP Professional x64 Edition.
hth