Try the following steps:
1. Go to Microsoft website to fix install and Unistall problems. Click "Run now" from Fix it to remove all iTunes & related installer files:
Be aware that Windows Installer CleanUp Utility will not remove the actual program from your computer. However, it will remove the installation files so that you can start the installation, upgrade, or uninstall over.
2. You should remove all instances of iTunes and the rest of the components listed below:
it may be necessary to remove all traces of iTunes, QuickTime, and related software components from your computer before reinstalling iTunes.
Use the Control Panel to uninstall iTunes and related software components in the following order:
- Apple Software Update
- Apple Mobile Device Support
- Apple Application Support (iTunes 9 or later)
Follow the instructions from Apple article listed here: http://support.apple.com/kb/HT1923 to remove all components
3. Reboot your computer. Next, download iTunes from here:http://www.apple.com/itunes/download/ and install from scratch