It may help to repair Apple Software Update from the programs and features control panel before attempting to reinstall iTunes.
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. Right-click on the setup file and run the installer as administrator. It may also help to pre-install the Microsoft Visual C++ 2005 Redistributable. In some cases you may also need to temporarily disable any anti-virus software.
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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