I don't think you have enough room on the drive to defrag your large files. Other than deleting those files, which I assume you want to keep, an external hard drive or an additional internal drive may be the easiest solution.
Also, there are many other possibilities for you computer slowing down. While I do defrag computers on a fairly regular schedule, once every month or so, I don't think I've never seen a defrag make a noticeable difference in speed, maybe a little in a severely fragmented drive but not dramatic.
Post how you progress
Monton