In case you have multiple partitions you move your data to a partition other than the one that has the operating system. You then format the partition with W8 and install the OS of your choice.
If your hard drive is formatted into a single partition you can try and create a new partition by shrinking the original and use the empty space to create another. Otherwise you can use a large usb drive or an external hard drive. Move data etc as is 1st option.
In both cases moving data will only work for movies,pictures, songs etc but not programs. You will need to install all your programs after you instal your new operating system.