Rip it up and start again...
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.
As given previously, manually copying .dll files from one path to another may occasionally serve as a workaround (assuming you even have the right version of the right .dll in the wrong place to copy) but otherwise it doesn't help and may cause further problems.
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