The downloaded file has most likely become corrupted. This can happen on rare occasions.
VERY IMPORTANT: You have to delete the file (otherwise the same error will repeat itself every time you download the software.)
Please follow exactly these steps to download correctly, do nothing else. You will only need to do this once, if you follow the steps exactly you will not need to do it again.
Step 
Create a new folder on your desktop, here you will save the download copy.
Step 
Copy and Paste the download address provided for your software into your internet browser window and press return. Do NOT type it in.
To Copy - Select the full address and Press CTRL +C
To Paste - CTRL + V
E.g http://softwarecasa.com/xxxxxxxxxxx
These are tested and verified files so we know the software is working properly.
Step 
VERY IMPORTANT
When starting the download you must NOT choose Run, you must choose "Save as" otherwise you will get an error again.
Choose to save the file in the folder you created in step 1.
Step 
When the file is fully downloaded, just double-click to start installation.
That's all there is to it.