Hi Rob,
Another user had the same issue a couple of weeks ago - can you please try the following to check the integrity of the zip file:
Open a Windows command prompt, no need to be administrator.
Switch to the folder where the zip file is, so probably 'cd Downloads'.
Run this command (assuming this is the correct filename): certutil -hashfile pf3_full_installer_3.16.0.zip MD5
This command will print a long MD5 hash string like this:
MD5 hash of pf3_full_installer_3.16.0.zip:
70d002dddc8dbcb2c8d7b32f3ab64a24
If the value printed on your system doesn't exactly match the value given here, then the zip file hasn't been downloaded correctly.
In that case you will need to try the download again until the MD5 hash matches - try turning off your AV temporarily.
(I also had this problem when I needed to reinstall PF3 the other week - the file download seemed to complete, but was actually faulty).
Hope this helps,
Martin
Edited at 16:18 - I gave the MD5 for the wrong file