If you got a corrupted download, the installer wouldn't be able to proceed, since it has self integrity checks, and you'll see a totally different error, clearly indicating the installer was corrupted.
The only possible reason why you can see the "not valid Win32 application" error, is because the antivirus has corrupted the file locally. Or, of course, if you really don't have any antivirus, as you said, a real virus might have done the same.