Vista - Windows Installer has stopped working

I received a message from iTunes to reinstall the software. So I downloaded
the latest version of iTunes ; but when I tried to install it I got the
following message:

"The Windows Installer Service could not be accessed. This can occur if you
are running Windows in safe mode, or the Windows Installer is not correctly
installed. Contact your support personnel for assistance".

I have been running iTunes on a Dell Computer with VISTA Home Premium for 3
months without problems. This problems just occurred today when I tried to
access iTunes.
I checked Microsoft Services and it showed Windows Installer as "stopped".
Could someone please help?

Thanks