With windows 8 release due in 2012, you can have a play with the new version for free before it's released.
You will need to download the Release To Developers version which is free to install. This is not a Beta version and the software may change before the final release, also I would not recommend installing it on anything other than a test PC as it is not an upgrade and once installed you cannot revert to a previous installation of Windows. If you do not have a spare PC with the right specifications you could always install it on a virtual machine for free and play with it there without sacrificing a current windows installation.
If you intend to use a Virtual Machine you can install Windows in the Oracle VirtualBox which seems to be the preferred choice, however I could only get it to install if I created a new IDE virtual hard disk as it would fail if I used the host Sata controller disk. (After creating the IDE VHD, Virtualbox may fail to respond. If you click cancel on the error reporting and re-open the Virtualbox software, your new Virtual Machine should be there). I also tried the free VMware Player and Microsoft Virtual PC but both produced "monitor errors" during installation, a common fault according to my research on the web. I did manage to install it on VMware Workstation version 8 which seems to be the only version of VMware which currently supports Windows 8.
You can download a trial version of VMware Workstation 8 for a month which gives you time to have a play with the new Windows Tiles etc. or you can download Oracle VirtualBox for free use. You will need to download Windows 8 as an ISO file, mount it in the virtual machine and then install it; or alternatively you can burn it to a DVD and install it into a virtual machine that way. I did the latter and it worked for both VMware Worstation 8 and VirtualBox (IDE VHD).
The installation is straight forward providing you have a PC capable of running the Virtual Machine and the new version of Windows, any PC running Windows 7 should be able to cope.