Hi John,
thanks for your reply. The link didn't help in my case. It's a bit out-dated also since it refers to a previous version of VS and none of the links it contains works anymore.
Meanwhile I found the solution accidentally. The different drives are not the problem. It is one of those privilege problems. After I had executed the C1 setup program under a privileged account, I continued working in my less privileged account as usual. However, the help installation is completed when the help collection is used the next time, showing a progress dialog box saying something like 'the system is doing something to reflect your recent changes'. This seems to have been unsuccessful without the privileges (unfortunately without any notice). After I had changed the privileges for the first call of the integrated help from VS the 'reflection dialog' appeared again, and now the help is complete again with the C1 section added.
Privileges, by the way: the C1 samples are installed in subdirectories of the program directory where C1 is installed. Since a less privileged user normally has no write access to the program directory, there are errors when trying to run the samples. I think that the samples should be installed elsewhere, or the setup program should ask the user where to install them.
Matthis