I have solved this problem.
R. Proffit is correct the error message (in english!) means that when the application tries to install it is unable to get to the proper place to install itself due to an incorrect registry entry.
The registry entry in question is the "rule-book" tell in the application where to go to install itself so if the rule gets damaged the application cannot install.
I changed the offenging registry back to the correct value and it fixed the problem.
The question I have now, however, is how did the registry get changed in the first place - have run Spybot, Ad-Aware, Malware and run McAfee up to date as a subscriber all the time. I got one alert a while back that a trojan had been intercepted on a webpage - maybe that?
Also where could I go to get an idea of what totally clean and "uncorrupted" registry entries look like.
I still ahve a weird looking reg entry in user shell folders.
Any and all help is greatly appreciated.