Start with: How to restart the Apple Mobile Device Service (AMDS) on Windows - Apple Support.
For general advice see Troubleshooting issues with iTunes for Windows updates (as given earlier).
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 theiTunes 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.
The section Install missing components has advice on breaking down the iTunes installer into the individual .msi files which might prove useful if a particular component such as Apple Mobile Device Support won't install normally.
Your library should be unaffected by these steps but there are also links to backup and recovery advice should it be needed.
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