In principle QuickTime is no longer an integral part of iTunes, and with up-to-date hardware and drivers it may not be required at all. Where it is necessary to support some video content then if you have a 64-bit system you may find that you also need to use the alternate version of iTunes found here: iTunes for Windows (64-bit - for older video cards).
See also Troubleshooting issues with iTunes for Windows updates. The steps in the second box are a guide to removing everything related to iTunes (including QuickTime) and then rebuilding it, which may be needed to get your system in order.
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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