Did the message say to install or enable JavaScript? The download process is using Java to detect your operating system for the correct version of McAfee. The process stops because you either don't have Java installed or enabled.
You can verify in Windows Control Panel if Java is already installed. There should be an icon (steaming cup of coffee) in your Control Panel.
If it's not installed, go to the following to download: http://java.com/en/download/manual.jsp
If it's installed, go to the following to learn how to enable: http://support.microsoft.com/gp/howtoscript
After you install and enable the Java Platform, the McAfee download process will detect the correct version to install.