Just highlighting this kb (2438651) that has been updated recently.
If you receive any of the following error messages
- The Windows Installer Service could not be accessed.
Windows Installer Service couldn’t be started.
Could not start the Windows Installer service on Local Computer. Error 5: Access is denied.
Then KB2438651 (How to troubleshoot problems that may occur when you install, uninstall, or upgrade a program on a Windows-based computer) may be of use to you.