You can use Wubi, just don't try to do so by unpacking the ISO image like an archive. Instead, you can download Wubi.exe from the Ubuntu website (where it is called "the Windows installer"), then put it in a folder with the ISO image. This folder should contain just two files--Wubi.exe, and an Ubuntu ISO image for the same version of Ubuntu. Then run Wubi.exe, and it will install Ubuntu, using data from the ISO image.
This was essentially answered by bcbc in a comment:
You lose me at 'unzipping the file'. You can either download wubi.exe
and run it, or the Desktop ISO. You don't unzip the ISO although some
archival software will open ISOs for you. If you have the ISO, then
download wubi.exe (make sure it's the same release) and place it in
the same folder as the ISO and run Wubi.
Or you can run Wubi.exe without the presence of an Ubuntu ISO, and it will automatically download the Ubuntu ISO image and use the image it downloads. (This might be seen as unnecessarily wasteful and slow if you already have the ISO image you need, though.)