If the process of using the System Configuration Utility to modify the Startup Selection requires you to de-select the Load System Services, an unavoidable consequence will be the disabling of the Windows Installer service.
In the event of the problem under investigation requiring subsequent use of the Windows Installer service, it may be started manually as follows.
Right-click My Computer, and then click Manage.
In the left pane, expand Services and Applications, and then select Services.
In the right pane, right-click Windows Installer, and then click Start.