Are you referring to Vegas or DVD Architect here? If you're referring to DVDA, then I think you mean "widescreen" rather than "hd." DVD Architect doesn't even give you the option of high definition because, as yet, there are no high definition DVDs. (Meaning, formats are being discussed and experimented with, but there's no standard for them yet.)
That said, if you do mean widescreen...how "thin" are the black lines? If they don't even extend inside the action-safe area, I wouldn't worry about it. If they do, are you sure your video is 16:9 or is it a 4:3 video that's been matted to look like 16:9?