I just succeeded in making work Terragen correctly. Which generates quite some answers on questions as well.
Warning messages about missing drivers were not there. I installed 2 times, one as every user, one only for my rights. No difference.
So both these aspects weren't the problem.
The difference was made by the choice of the directory Terragen should go into.
On the W2K set I was used to have Terragen installed in a dedicated "terrain" subdir on another partition.
Now, I have only one partition, but I still tried to install TG in such a subdir so far.
This time, I let the setup-procedure do its default ways, so TG ends up in the Program Files directory. And then it works..
I'm very happy of course.
On the other hand, I wonder. Is this path-restriction a buggy behaviour, or is this still due to the XP organization ?