Windows Installer Service and Vista UAC not working

My wife's laptop uses Vista Home Premium/SP 2/32 bit. Avast anti-virus. Advanced SystemCare 3 is also loaded, but as far as I can tell is not in conflict with the anti-virus program.

TimW at the Malware Forum referred me to this forum, having ruled-out malware as the probable cause of the problems.

The laptop won't download any Windows Updates nor many other programs; the error message says the Windows Installer Service is not running/cannot be accessed. I followed a MS forum suggestion to start the Installer Service via "Services and Applications" in the "Manage Computer" console tree, but the problem remains.

A Windows Security alert says that the Windows Firewall is turned off; and I get an "Access is denied" message when I try to turn it on. As far as I can tell there are no other software firewalls running.

I can't access the "Add or remove user accounts" to create a new account or edit my wife's existing user/administrator profile.

Any help you can offer would be very much appreciated.