Microsoft unveiled Windows 8.1 Preview at their BUILD 2013 conference on Wednesday, where it's free and some people can get their hands on it right now. Microsoft have promised an ISO image of the OS, but for now it's only available through the Windows Store.
This means the Preview of Windows 8.1 is currently locked down to those running Windows 8 and Windows RT - so not me then. Remember, that this is just a beta - so don't replace your full OS when using it. Maybe partition your hard drive, or use a spare one (if you have one). The Windows 8.1 Preview should be unleashed to all of us in the coming days, and we'll write up a guide on how you can install it in our Tweakipedia section once it goes live in ISO format.