Try repairing Apple Software Update from the Programs and Features control panel, then uninstall it before trying to reinstall iTunes. If it cannot be found or won't repair try the MS install & uninstall utility mentioned in note 1 in the following guide.
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. Some users have had to completely remove AV software in order to get iTunes to install.
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.