17% free space should not be an issue for you, although if your computer is very old it may run a little slower.
However if you are down to 17% it is likely that you will continue to fill the rest of the space, so you can either remove software and files that you are not using, purchase an external drive or look into online storage options to free up some space on your PC.
The best thing to do is to add another hard drive whether it is an internal drive (inside your PC) or external drive (has it's own case and power supply).
If you don't want to spend any money, then your only option is to uninstall rarely used programs and delete any unimportant files. Afterwards you should defrag your hard drive just to keep things nice and tiddy.