You say your hard drive has 15GB of spare space left on it. I know some videos and some software programs are very large nowadays, but I can't recall ever seeing anything as big as 15GB
Most software you install or any videos you purchase and save to the hard drive surely are not going to exceed 1GB, or 2GB at the most. So I am unsure why you say the files you are downloading are larger than the free space you have.
Is there some confusion here?
Why not purchase an external drive and move some of your existing personal files such as videos, photos, music, etc, to that, and free up more space on your hard disk? Don't forget you need to back up thos important, cannot be lost files as well, because hard drives whether internal or external can fail.