Software Update

• Glitches in older versions of iTunes, Outlook and the iPhone's operating system (iOS) have caused issues with calendar synchronization. Download and install the latest update for Outlook and version of iTunes through Windows Update and Apple's website, respectively. If you are still having trouble after acquiring the latest update for your version of Outlook, you may need to purchase the latest version from Microsoft. Click "Check For Update" in the "Summary" tab for your iPhone in iTunes to update the phone's operating system.
  
  

Configuration

• You must correctly configure iTunes to synchronize your iPhone with Outlook. Select your iPhone from the devices menu in iTunes while it's connected to your computer. Click "Sync Calendars From" in the "Calendars" section of the "Info" tab, then select "Outlook" as the synchronization source. Select the calendars you want to synchronize and click "Apply."
  
  

Sync History

• In some cases, the calendar information in Outlook may overlap with the information in your iPhone, preventing it from synchronizing properly. If this happens, reset the sync history from the "Devices" tab of the "Preferences" menu in iTunes. The program then prompts you to merge the data on your iPhone with Outlooks' data or replace it next time you sync.
  
  

Add-Ins

• Some third-party Outlook add-ins may prevent iTunes from syncing your iPhone's calendar with Outlook. Disable all the add-ins in the "COM Add-Ins" portion of that list in Outlook's options window. If you like, you can enable the add-ins one at a time and re-synchronize your calendar to try and determine which ones are preventing iTunes from synchronizing with Outlook.
  
  

User Account

• Some of the calendar entries in Outlook may be corrupted, preventing proper synchronization. You can create a new Windows user from the "User Accounts" window, which is accessible from your Windows' Control Panel. Note that the new user will not have the same settings as your main user account. You may need to search for iTunes and Outlook in the start menu and reconfigure Outlook. Once you're able to do so, create a new calendar under the new account with several events. Attempt to synchronize this calendar with your iPhone. If you're successful, log in to your original account and remove existing calendar entries until your iPhone synchronizes with iTunes and properly updates your calendar.
  
  

Software Reinstallation

• In some cases you may need to uninstall Apple Mobile Support, iTunes and Outlook from the "Programs and Features" section of your computers control panel to repair a corrupt installation. Once you have completely removed these components from your system, download and install the latest version of each program. You may need to purchase the latest version of Outlook, if you do not already own it. Upon re-installing these programs you will need to reconfigure your settings including your email account information in Outlook and your iTunes library and sync settings. Your iTunes media library will not be deleted when you uninstall iTunes, however Apple advises that you backup your media collection from your computer to an external hard drive as a precaution.