1. Are you able to open the zip file i.e with WinZip to check to see if it not be corrupted at some point?
2. Do you do anything funky within your environment where by your users temporary folders locations are redirected to a network location?
There were no "funky" stuff done on the clients.
The only thing that I have done with the downloaded ZIP file was clicking properties and selecting "UNBLOCK" button.
I have managed to solve this problem with re-downloading it and without clicking that UNBLOCK button under properties.