7-zip: "Cannot open file as archive"?

Whenever I try to extract a certain file, I get the message "Cannot open file '(file name)' as archive. I hope it's not corrupted. This thing is over 4 gigabytes and takes 5 days to download. Is there any free program of technique that can fix this?
Update: Well... the file is an ISO. file. I'm not going to play it or anything. I want to rip stuff from it an put it into SSBB.
Update 2: BTW, it's a Mario Kart Wii ISO. I think WackaAlpaca said something about it being an arc. file.
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