Try repairing Apple Software Update from the Programs and Features control panel, then reinstall iTunes. If it won't repair try the MS install & uninstall utility mentioned in note 1 in the following guide, and if that won't work try the Missing .MSI advice.
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.
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, need to revert to an older version or want to try the iTunes for Windows (64-bit - for older video cards) release as a workaround for installation or performance issues, or compatibility with QuickTime or third party software.
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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