Windows Installer cannot be accessed

XP SP2

This happens when I try ti install one of two different security suites: Comodo and AVG.

I have read and tried http://support.microsoft.com/kb/324516. That includes reinstalling SP2.

I have used the Clean Uninstall for Comodo and Avast, on top of the default uninstall utility for each programme.

A possible clue is that even after standard and clean uninstall, a _third_ Suite (Zone Alarm) says that it cannot install on a system where Comodo is already installed (I had Comodo installed before).

Another thing is that when I try to search the registry for HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSIServer, or even just "\msiserver" I get no find. That has to be wrong, but how would I fix it?

Any other ideas, please?

Any help greatly appreciated.,

Nick