Installing Windows 8 is a bit different than installing the previous version of Windows. Windows 7 you could install without entering a product key, but when you install Windows 8 you are required to fill in a key right-away. For a normal installation that doesn't matter much. But if you only want to experiment with Windows 8 for testing purposes, or for any other reason only will have it installed on your computer for a brief period, then it's a different question.
A quick patch update to Windows can extend the useful life of a PC. To take the fear out of Upgrading Windows 98 to Windows XP, rely on this guide. - PCSTATS Tips