On Windows there's the added problem of defragmenting, which moves files around and only "deletes" the old copies in the sense of removing the pointers. They still exist on disk. I agree the mere presence of Tor isn't illegal and wouldn't worry about it as long as you didn't save a link to SR in your bookmarks, but OP asked how to remove it anyway. I firmly believe that a random write across the whole hard drive is the only safe way to remove all potential traces of a file. "Shredding" programs that overwrite individual files multiple times are an illusion of safety, since they only overwrite the most recent copy of the file, that the OS knows about.