Before you upgrade to Windows 10, do either an Image of your hard drive as Trouble said OR get yourself a second hard drive with enough capacity for your needs and install Windows 10 on that one. You have to buy Windows 10 now anyway as the freebies are over (unless there is a way I do not know about).
I for myself have the original hdd's of my laptops stored in a closet and installed what I wanted on them on a separate drive.
Since m laptops are already close to 10 yrs old I have used a SSD for the OS and replaced the optical drive with a hdd caddy (you can find those on ebay), so I have my personal files on the second drive and was able to get a small SSD (lower price) for Windows...