You want to make a DUAL BOOT (Windows 8 and Windows 7 - 64 bit) on your hard drive without having to erase your current data, this can be achieved ona computer with a UEFI motherboard and hard drive in the GPT format:
To begin, disable the secure boot feature in the BIOS of your motherboard and enable BIOS legacy mode.
You will then need to change the UEFI boot order, putting the CD/DVD ATAPI drive first.
Create a partition
Right click on your "My Computer" and select "Manage".
A new window should appear. In the Disk Manager search for "Disk 0" and select the partition you want to divide (normally the one on which your current OS is installed)
Right-click > Shrink.
Select the size of your new partition.
Your new partition will appear as unallocated space.
Right-click the partition and select "New Simple Volume", which should bring up a new wizard to create a simple volume.
Choose a drive letter (G:, H:, etc ...) and follow the onscreen procedure to finalize the creation of your new partition.
The commands to be entered
Open the command prompt (Run > type "cmd") and run the following commands:
Type "DISKPART" and confirm with Enter.
DISKPART should then open, type "SELECT DISK 0" a and confirm with Enter.
A message stating that disk 0 is selected should be displayed.
Now type DETAIL DISK and press enter: The volumes present will be listed.
Then locate the volume that you created earlier and enter "SELECT VOLUME volume_number" and press Enter.
DISKPART would then indicate that the volume has been selected.
Type AUTOMOUNT and press Enter
DISKPART will then tell you that the automatic mounting of new volumes enabled.
Finally you need to type EXIT to exit DISKPART and close the CMD window.
Now you should be ready to install a new operating system.
Installing the new OS
Note that: you must use the 64-bit versions of Windows for GPT formatted drives
Simply insert the installation DVD of Windows 7 and run from the desktop. During installation, you must choose the volume that you have created earlier.