1024MsiInstallerProduct: Adobe Reader 9.3 - Update '{AC76BA86-7AD7-0000-2550-7A8C40000934}' could not be installed. Error code 1625. Windows Installer can create logs to help troubleshoot issues with installing software packages. Use the following link for instructions on turning on logging support: http://go.microsoft.com/fwlink/?LinkId=23127
On Windows 2008, with UAC enabled, the Adobe Updater doesn't launch elevated when you click the notifier on the task bar, and thus the installation fails.
You can launch the update manually, by starting C:\Program Files (x86)\Common Files\Adobe\ARM\1.0\AdobeARM.exe manually. However, make sure you right-click the application and select "Run as Administrator".
You can also launch the Adobe Reader elevated (e.g. right-click the desktop shortcut and select "Run as Administrator") and then select "Help -> Check for Updates". This will also launch the update utility elevated.