How to Dual Boot Windows 8 and Windows 7
STEP 2: PARTITION
You'll have to create a partition of at least 16GB (20GB for 64-bit Windows 8) for your side-by-side Windows 8 installation. To do this, type "disk management" in the Start button's text box, which will display a "Create and format hard disk partitions" choice at the top of the Start panel. Click on that to open the Disk Management utility. You'll probably have two partitions. Right-click on the largest one, and choose Shrink Volume from the context menu.
A "Querying Shrink Space" dialog will appear for a while, then another message will tell you how much free space can be squeezed out of the drive in MB. Enter a size above 16GB for the 32-bit version of Windows 8 and over 20GB for the 64-bit flavor, and then hit the Shrink button. This will create an Unallocated section equal to the size you chose in the chart at the bottom of the window. Leave it be for now, we'll let the Windows 8 installer take over from here.
STEP 4: RUN THE WINDOWS 8 INSTALLER
Pop in the installer DVD you burned or the USB stick you prepared, and restart your PC. Choose your language, then "Install Now." The product key is on the ISO download page (for reference, it's TK8TP-9JN6P-7X7WW-RFFTV-B7QPF). Accept the prerelease software license, and after this, choose Custom and not Upgrade. Now is when you're presented with the choice of partitions, click "Drive options (advanced)" and then select the Unallocated space we created in step 2. Next, click New from the drive options icons below. Accept the full size displayed, and hit Apply.
Now we've got a fresh partition to on which to install Windows 8 onto. Hit Next, and setup is off and running. Go have a coffee or other snack of your choice; it takes about 20 minutes. Note there was no choice for specifically creating a multi-boot setup—since you used a separate partition, that will be enabled automatically.
By default, this screen displays for 30 seconds, and then Windows 8 launches. To change that default to your older OS, choose "Change defaults or other options" from the bottom of this multi-boot screen. Here you can change the timer value and the default boot operating system. You can also use troubleshooting tools like Refreshing or Resetting your PC, and you also have access to advanced options like system recovery and a command prompt.