ADDITIONAL FIX - IF ALL ELSE FAILS
(For Windows computers - not sure if it's the same for Mac but worth a try)
If all the above still doesn't work (and everything else you try that you have read on the internet like me) I have just spent an hour on the phone with a senior support tech from iTunes and finally found the problem.
1) Firstly I uninstalled everything 'Apple', see here - Remove and reinstall iTunes and related software components for Windows Vista, Windows 7, or Windows 8 - Apple Support
2) Then go to 'C:\Program Files\Common Files\' and delete the 'Apple' folder if there is one. Do this also for 'C:\Program Files (86)\Common Files\'
3) OK now here is what I did differently. Open your file search 'C:\' (click the Start Menu --> Computer --> OS (C:). In here search in the top right for the following file 'msvcr100'. It is a .dll file.
4) Click the one titled exactly 'msvcr100' (there might be another one with a longer name ignore that one). Drag the 'msvcr100' file to the desktop, DO NOT delete or copy it (yet).
5) Now go to iTunes.com and download the latest iTunes (probably save it to your 'Downloads' folder). Once downloaded RIGHT CLICK on the newly downloaded setup file and click 'Run as Administrator' (this is important).
6) Follow the usual iTunes installation guide and it SHOULD install OK. If it does then you can now delete the 'msvcr100' file from your Desktop.
If it doesn't work then I would get iTunes Support to ring you. Here's the link to the support - https://getsupport.apple.com/
Hope this helps people! Good luck 🙂