This means you must have a corrupted download because, it should happen exactly the opposite, since the installer IS digitally signed with a valid digital signature, you should see the message that tells you that a program that has been digitall signed is trying to be executed, with the yellow shield.
Instead, if there was not digital signature at all, it would have said that a program from an unknown source is trying to be executed, with the red warning.
So, if something says that there IS a digital signature, but it's invalid, the only explanation is that the signature itself has been corrupted, probably because the download is.
Try to clear your browser cache and download it again.
Also, don't execute the file directly when downloading, but save it on the hard drive first, then execute it.
