You're trying to download iTunes onto your Windows Vista computer, and your getting this message, "The Windows Installer Service could not be accessed. This can occur if you are running Windows in safe mode, or if the Windows Installer is not correctly installed. Contact your support personnel for assistance".
Do you need to download a new installer? You tried downloading an iTunes compatible version for Windows Vista, but received same message. You can try disabling User Account Control.
Click your "Start" button then select "Control Panel". Type in “UAC” into a search box. You will see a link for “Turn User Account Control (UAC) on or off”.
On a next screen you should not check a box for “Use User Account Control (UAC)”, and then click on an OK button. You will need to reboot your computer before those changes take effect, but you should not see that message anymore when attempting to install iTunes.
Once you have iTunes installed successfully, then you can go back and enable User Account Control.