Try to redownload the file using a download manager such as Flashget or Free Download Manager. That's usually the case of bad download connection.
Does it say something like CRC error or something when you're trying to extract them ? Or try to use the latest winrar.
I don't really recommend this but..... on the extraction options, you can tick the choice "Keep Broken Files" to force the winrar to extract the data. I did it before and there doesn't seem any problem with the ISO that I extracted that way, but it's not really guaranteed though.
It means either your download is corrupted or your Extraction program is outdated. Update your WinRar/WinZip program, and if it still happens, it's because your download is being corrupted/unfinished. Use a good download manager like FlashGet.