Typically that error is caused by a sticky copy of Apple Software Update that won't remove itself when requested. Try repairing that first. Then try installing iTunes 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. Right-click on the setup file and run the installer as administrator. In some cases you may also need to disable any non-Microsoft anti-virus software while installing.
Review the other boxes and the list of support documents further down the page in case one of them applies.
The further information area has direct links to the current and recent builds in case you have problems downloading, or want to revert to an older version if the current one won't work properly for you at the moment.