How large is the file in kilobytes on your hard drive? You might be getting this error because the entire installer file did not finish downloading.
After reading your message I downloaded the installer fresh from the site and installed it on a computer in my house that didn't have the synther installed yet. It installed perfectly for me, so it seems like the file on the website is still good.
When I right click on the installer file and click 'Properties', it says that the file size is 10.9 megabytes or 11,488,096 kilobytes. If you right click on the installer and see a number smaller than what I reported, I would suggest that you try to re-download the installer.
Here are two links for the official installer (Right click on either one of these and 'save target as'):
Let me know if you still have trouble.