The iTunes for Windows application should have all the needed decoders built-in. The installer would check for minimum system requirements. Verify that your app version is up-to-date (v12.13.3.2). However, iTunes is being ‘sunset’; no further development.
What happens when you try to watch a purchased movie in the iTunes application? Do you get an error message? What does it say exactly?
For Windows 10/11 users, there is now a replacement app, the Apple TV app. You can’t run it side-by-side with the iTunes app; installing the newer blocks the older one. Possibly to preserve library integrity.
iTunes user guide for PC - Apple Support
Apple TV app user guide for Windows - Apple Support
Use the Apple TV app, Apple Music app, and Apple Devices app for Windows - Apple Support
Unlikely iTunes, this Apple TV app is just for video libraries. For other functionality, there are separate companion apps as well: Apple Music app and Apple Devices app. These are intended for all computers that can run these. Only (older) incompatible computers should be using the legacy iTunes application (v12.13.3.2), or that is what Apple would like to see for app adoption.