If the user cannot install anything with Window Installer Service, so to fix this problem you sold manually start the Window Installer Service or you can re-register the service with Command Prompt. If in case, you need more help, then go to www.office.com/setup. 

Method To Fix If Window Installer Service Could Not be Accessed on Window 10:

1. Reset Windows Installer Service Registry:

To reset, you should hit on Win Key + R together and then input regedit and tap on Enter or OK button. When Registry Editor opens, you should choose File option and then hit on Export option. Then, you have to choose All option in Export range and then input the name of the file and then tap on Save option. Keep in mind you should run this file and just restore your registry. When you created your backup, then close Registry Editor. If registry backup is created, then you should create a .reg file and then add it to registry.

2. Start Windows Installer Service Manually:

To start the installer, you need to tap on Win Key + X together and then select Command Prompt as Admin from the menu. When Command Prompt opens, you should enter net start MSIServer and then hit on Enter key to run it.

If the user wants to check that Windows Installer Service is running, then they should tap on Win Key + S at the same time and then input Services. Here, you should choose Services option from the list of results. If Services window opens, you should find Windows Installer service and then double tap on it. At this point, you should check the Service Status to check if Windows Installer service is running. If in case, the service is not running, then tap on the Start button in order to start it.

3. Re-register Microsoft Installer Service:

For this, you should tap on Win Key + X together and then choose Command Prompt from the menu. If the user have 64-bit version of Windows, then you should enter the below lines and then hit on Enter key after each line:

%windir%system32msiexec.exe /unregister

%windir%system32msiexec.exe /regserver

%windir%syswow64msiexec.exe /unregister

%windir%syswow64msiexec.exe /regserver

When you enter all the lines, then close Command Prompt and just restart your computer. In case, the user have 32-bit version of Windows 10, then enter the below commands:

msiexec /unregister

msiexec /regserver

At last, when you enter the commands, you should close Command Prompt and then restart your system.

4. Use regsvr32 command:

If the user uses this command, then they should start Command Prompt as an administrator. When the Command Prompt begins, you should enter regsvr32 c:windowssystem32msi and then tap on Enter key to run it. When you run the command, you should close the Command Prompt and then again install the problematic application.

5. Use SFC and DISM Commands:

You should tap on Win Key + X simultaneously and then select Command Prompt as Admin from the menu. If you do not find the Command Prompt, then you should use PowerShell (Admin). If Command Prompt begins, then enter sfc /scannow and tap on Enter key to run it. SFC scanning process will take 15 minutes, so you should not interrupt it.

When the scanning process finished and if the problem is still there, then you should use DISM scan. For this, you should start Command Prompt as administrator. If Command Prompt begins, then run DISM /Online /Cleanup-Image /RestoreHealth command.

Here, the scanning process will start and it will take about 20 minutes.

6.  Install Windows Installer Redistributable:

For this, you should download Windows Installer Redistributable. When the file is downloaded, then double-tap on it to begin the installation process. At last, follow the instructions to finish the process.

With this method, you can fix Window Installer Service could not be accessed on Window 10. For info, visit to office.com/setup or office.com/myaccount.