Well, as Örjan said it usually seems to be winrar upon installing taking over the fileassociation for isos, wasn't sure about renaming to zip, probably you can set the preferred extension within winrar, but I've read quite a few posts here about the iso -> rar problem. Wouldn't surprise me if WinZip did the same.
Besides, I wasn't flaming windows or winrar, I was just stating something that's happened to others.
When you install winrar IIRC it asks which filetypes should be associated to it but as default most are checked.