Backing up of data

Hello,

I have this data of about 70 gb in my hard disc distributed in 3 different drives. I usually format my C drive to reinstall my OS. I have never formatted my other drives. Coz there is data which I require like, movies, songs, wallpapers. I dont want to lose them. I even dont want to back up them in many separate DVDs, or any other device, coz i dont ve them or i cant afford them. So can u tell me a way to format my hard drives without losing my data?