What does "invalid MS-DOS function" in Windows 10 mean, and how do you fix it?

I’m not really a big Windows fan, but I have it installed on my gaming laptop, and it is updated to Windows 10.

About a week ago, I started getting “invalid MS-DOS function” errors when trying to delete a single file… a .css file. I didn’t think much about it until a second file just now started giving me the error. The second file is a .avi file.

What does this error mean? And is there a way to fix it?

Another, maybe related, problem is that Dreamweaver has started telling me it can’t write to certain tmp files that don’t even exist when I try to save my documents.