If you own a Windows account already, (which you will have needed to download the Windows 10 Technical Preview) you will probably know that you can use this to sign into your virtual PC. You may use this for Window 8 already, or your Xbox 360 or Xbox One, or Windows Phone.
With Windows 10, signing into the computer with your usual account will result in changes being synced to the cloud. There seems to be some sort of hierarchy of versions which will mean that the changes made to your Start screen (such as the chosen theme colour) will reflect on your Windows 8 Start screen, so be careful with the choices you make.