Gosh, that's interesting.
So even though you said this is happening to apparently random files, it's happening repeatedly to those files when you try the saves multiple times?
Gosh, my friend, these specs are a real worry, and could go a long way to explaining the problem. That's such a small hard drive to begin with, and not much free space to allow your software enough working room. I urge you to run Glary Utilities right away.