When a new operating system is released, many users prefer to keep their old one running alongside it just in case there are any problems with the new installation. This is strongly recommended when using a pre-release version of an operating system such as Windows 8 Consumer Preview. Windows 8 makes it easy to dual boot into two or more operating systems thanks to its provision of a boot manager utility. Here is how it’s done. The following assumes that you already have an empty partition or hard disk to install Windows 8 on.
Insert the Windows 8 DVD or USB flash drive and restart the computer. Ensure that your BIOS is configured to let the computer boot up from a bootable flash disk or DVD. On most computers, you can access the BIOS by pressing the ‘DEL’ or ‘F2’ key on your keyboard. You should be able to find the boot device priority settings under the ‘Boot’ menu of your BIOS. Restart the computer and wait for the Windows 8 setup screen to appear.
Perform a clean installation of Windows 8, installing it onto an empty hard disk or partition. You can also install Windows 8 to an unallocated space on the disk. It will automatically create the necessary partition for you. Take care not to install Windows 8 on the disk which currently has Windows 7 or Windows Vista installed on it since doing so will replace the current operating system and cause you to lose all data on that partition. Continue though the installation wizard until the process is complete.
Once the installation is complete, Windows 8 will automatically be set as the default operating system and it will start after the wait time if you do not choose another option in the boot screen. When you restart the computer, you will be asked to choose an operating system. If you want to customize the boot screen, click ‘Change defaults or choose other options.’ On this screen, you can change the timer, specify the default operating system and choose from various other options. If you want to know more about this i recommends you to visit tech blogger. I know that this site will also give you lot of stuffs.