Review the other boxes and the list of support documents further down the page in case one of them applies. It may also help to pre-install the Microsoft Visual C++ 2005 Redistributable and/or the Microsoft Update for Universal C Runtime. When reinstalling right click on the downloaded setup file and use run as administrator.
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. For general advice see Troubleshooting issues with iTunes for Windows updates.