First make a back up of your important data, now we need to get a drive where we will install our Windows 8, for this :-
Create New Partition
Click on Start button then right click on Computer and select Manage,
Now click on the Disk Management option from Left Sidebar,
Now right click on any of the drive in which you had at lest 20GB of free space and then click on Shrink Volume,
Now free up 20 GB of space,
Now you will see another option coming as Unallocated space,
Right click on I and select option New Simple Volume and then click Next and Next when you see option Format Partition,
Just provide a name to your new partition and then click Next to format it.
Install Windows 8
Now you had New Partition space and you are ready to install your new OS,
Insert your DVD or attach your USB to your computer and boot computer from them,
Installation of Windows 8 is very much similar to as of Windows 7, select your language, make sure we are going to do a custom installation of Windows 8,
When it ask for partition where it wanna install Windows 8 select the partition we created above, if you select any other then you might loose your data.
Now grab a coffee and let Installer do its work and it will reboot your computer one or two times,
Once installation is done then it will show your Windows 8 Start Screen.
Make Windows 7 As Default
You don’t want to boot in Windows 8 by default as you don’t have much data on it and its not your primary OS, So when you first boot in Windows 8 you will find a graphical interface saying Choose An Operating System,
At the bottom of that page you will see an option saying Change Default Option, click on it and then click on Choose Default OS and then select your Windows 7 as default one.