I got this suggestion from their Help Forum if the first plan of action doesn't help to reduce the Disk Usage. My Disk Usage was too high because I needed to Troubleshoot Windows Update problems in the Troubleshoot menu selection of Control Panel.
The Windows Installer cache folder is a hidden system folder located at C:\Windows\Installer. Whenever you install a software using the Windows Installer, a copy of changed system information is placed in this folder. The Windows Installer cache contains the saved copies of the installed updates that are applied by the Installer
How to Fix the "Windows Installer Service could not be accessed" Error in Windows 7 - Have you tried to install a program in Windows 7 that uses an MSI file as its installer and instead you saw the above error? Never fear. There is an easy solution and we’re here to help you with it. | HTG