That means the download is corrupted, as in, you did not got all data or some data got damaged while you downloaded. It is always recomended you do not use the computer while downloading, to make sure.
What source did you download from, ModDB?
Yeah, been a lot of that lately; someone said ModDB was hacked at a point and files were affected, but was that the case I doubt ModDB would not solved it already, it's one of their most popular mods, naturally.
Rather I'd guess it's some anti-virus update that is set to warn for files that it's uncertain of, and some of those are apperently in TATW. However, the game created and uploaded by King Kong do not have virus, that 1½ million players over 4 years can certify ^^