Preview builds require the same activation support as any other copy of Windows 10. Either you upgraded from Windows 7 or 8 before the deadline six weeks ago or you need a retail license. Windows XP is irrelevant. At installation time the licensing software registers your computer as a Windows 10 device if you provide a product key during or following installation. Thereafter no product key is needed to re-install Windows 10. It doesn't matter now what OS is on the computer since all copies of Windows 10 now require purchase. If you have purchased Windows 10 and have a product key it doesn't matter what version or build you clean install just so long as you provide the product key when asked. This includes preview. Previews are full-fledged copies of Windows 10 but contain some beta features.