Even if you're excited for Microsoft's new OS, you might not be planning to buy a new computer. If you have an existing Windows PC, Microsoft has made the upgrade process more or less simple, but there are still some things you might want to think about before making the switch. Here are a few key points to consider. (We'll continue to update this post as more information comes in over the next few weeks.)
Read more at source:
Upgrading to Windows 8: What you need to know (FAQ) | Windows 8 - CNET Reviews
See also: http://www.eightforums.com/tutorials/2234-upgrade-install-windows-7-vista-windows-8-a.html