Ok, I just went and redownloaded 2 zips from a purchase I made earlier this month.  They are part 1 and part 2 of one pacikage.

I could open them no problem using Winrar and Winzip. All of the contents extracted without an error.

However, when I right clicked on the zip and picked "Compressed Zipped Folders" which comes with Windows, and tried extracting the runtime folder, I got a windows popup window error message saying the following:

"The system cannot find the file specified"

The way the "Compressed Zipped Folders" works is that when it opens up it looks like you are viewing files in a standard folder.  To extract the contents you just drag and drop to where you want.

When I tried dragging and dropping the runtime to another folder, I got that "The system cannot find the file specified" error, and the only files that were actually extracted to where I was dragging the runtime to was a .DS Store icon, the Renderosity License and the product readme.  None of the actual product files were copied.

I repeated the exercise with the marketing zip that was included earlier, and managed to open the zip with the "Compressed Zipped Folders" method and I was able to drag and drop the marketing folder to another folder without any issues.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi