key not valid for use in specified state

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 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.

The section Install missing components has advice on breaking down the iTunes installer into the individual .msi files which might prove useful if one component won't install normally. It might at least narrow down which component isn't installing right now, but unless we know which (registry?) key it is complaining about it may still not install. See this post for a method to generate log files during the install of the problem component: Re: AppleMobileDeviceSupport64.msi for windows 8.1

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