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.
Your library should be unaffected by these steps but there is backup and recovery advice elsewhere in the user tip.
If you are running 64-bit Windows and the above steps haven't helped try installing the version of iTunes that can be downloaded from this support document: iTunes 12.1 for Windows (64-bit — for older video cards). This is a 64-bit installer for the 32-bit core application, the same as all previous 64-bit releases of iTunes, which may well work better with third party tools expecting to see the 32-bit iTunes executable.
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