If it is, unblock it-- (if you see it) it is the Quicktime Task executable file, that may be blocked by the Windows Security rules.
To remove this should free up iTunes to function again
• If you do not see the QTTask.exe file is not listed
* Check the rules and quarantines lists within your Avast-Antivirus software as well.
I do not have access to that anti-virus so I am not sure how to access the properties of that software.
However I can say, typically, if the Anti-virus software has an icon on the system tray (bottom right corner on the task bar near the clock) you may be able to right-click and select the properties or advanced settings and check those settings there.
Let me know if this helps.
I will be watching for your update on this.
Thanks for your patience!