The iso files you usually have to burn as an imgage onto a CD/DVD or mount them with a tool like Daemond Tools.(use Google to find it)
For the zip and rar files, besides that it is possible that they're really damaged and/or corrupted, they might have been compressed with other compression programs and both winzip and winrar do not support those compression types.(use Google to find others)
Another thing could be that they're actually viruses/trojans and/or worms and that's the reason that they are not really open.......though that's just a guess.
