You DEFINITELY have to burn the iso file to DVD to get the choice to install to a different partition. When you insert the DVD into your reader and reboot you will have a choice of Upgrade or Custom install. Choose Custom install and you will get the choice of where to install. This is where you would choose to install to a different partition than your Win 7 partition.
May I make a suggestion. Before doing this, make an Image of your Win 7 Partition just in case something goes wrong. With an up to date image, if something goes wrong, it will take about 10 minutes to restore your Win 7 OS. Be sure to remember to make a bootable rescue media with whatever app you use to creat the Image.