Ensure you haven't renamed any files. If your files start at .008 , you are missing the beginning of the archive and cannot open it.
This usually means you are missing one of the parts (e.g., you have 001–006 and 008, but missed 007). Check your download folder. Y2K.7z.008
You must have every preceding part in the same folder (e.g., Y2K.7z.001 , Y2K.7z.002 ... through Y2K.7z.008 ). Ensure you haven't renamed any files