So you may not want to download the Windows 8 Developer Preview and take the time and trouble to see how that works on a PC.
Bot, you may want to check out Windows 8. What to do?
Windows 8 Simulator may be what you’re looking for.
This is a simulation tool rather than a replacement for your existing operating system therefore it doesn’t affect the copy of Windows that you are using. The simulator runs in its own sandboxed window that cannot affect any other part of your system.
Obviously it’s not a full version of Windows 8 but it does give you a rough idea of what to expect in the next version of Windows.
Disclaimer: I haven’t tried it myself yet (been too busy) but it has a good rep online.
You can find out more and download a free copy at the Windows 8 Simulator review page.