Thanks, Cavalcader, but I think you might have been in a hurry, and didn't read my full post. I wrote:
'So I clicked on "Launch application", and - nothing happened. For 10 minutes, until I gave up waiting. then I went to the folder where I had saved it and tried opening it from there. And got the same error message I originally started with - "file is not a valid Win32 application".'
So I have tried both of your suggestions, with no luck. That's why I'm giving up and am now looking for a program that will actually install on my computer.
BTW, in my experience Firefox does not always give a warning like this, which refers to an external app being launched - it usually just warns that the file is executable.