iTunes Update

You may need to start with a visit to the Microsoft troubleshooter at Fix problems that block programs from being installed or removed, which might be able to repair issues with the Windows Installer process.

For general advice see Troubleshooting issues with iTunes for Windows updates.

The steps in the second box are a guide to removing everything related to iTunes and then rebuilding it which is often a good starting point unless the symptoms indicate a more specific approach. When reinstalling right click on the downloaded setup file and use run as administrator. It may also help to temporarily disable any non-Microsoft anti-virus.

Review the other boxes and the list of support documents further down the page in case one of them applies.

The further information area has direct links to the current and recent builds in case you have problems downloading, or want to revert to an older version if the current one won't work properly for you at the moment.

Your library and device backups should be unaffected by these steps but there are links to backup and recovery advice should it be needed.

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This seems to be a windows issue, not an iTunes one.

You can try to download the latest installer and use that to update iTunes instead of the Apple Software Update program.