Very often when trying to install program which uses Windows Installer, in Windows Vista you can get the following error: 

The Windows Installer Service couldn't be accessed. This can occur if the Windows Installer is not correctly installed. Contact your support personnel for assistance.

This error usually happens in Normal Mode due to the reason that this service is disabled in Safe Mode.

There are few ways to get rid of this error:

A. Start the Windows Installer process in Windows Services.

1. Start > in the search bar at the bottom, type in: 'services.msc' without quotation marks. Press Enter.

2. In the opened list of the services find Windows Installer service. Right-click on it and choose 'Properties'.

3. On the General Tab set the Startup Type of Windows Installer to Manual.

4. Click Start button to start the service.

5. Click Ok.

B. Unregister and re-register the Windows Installer:

1. Start > Run(Step A.1.), type in MSIEXEC /UNREGISTER, and then click OK. Even if you do this correctly, it may look like nothing occurs.

2. Start > Run(Step A.1.), type in MSIEXEC /REGSERVER, and then click OK. Even if you do this correctly, it may look like nothing occurs. After you run this command, the operation is complete.

3. Try your Windows Installer-based application again.

C. Windows Installer Registry Fix (in case Windows Installer service is not listed in the services):

1. Download msiserver.zip and Save it to the PC.

2. Unzip the file and extract msiserver.reg to the Desktop.

3. Right-click msiserver.reg and choose Merge.

4. Click Yes to confirm the changes.

5. Restart Windows.