Dear Laszlo,
happy new year, and thank you very much for trying to help me!
You know, I'm not that type, who just asks, and waits for the miracle...
As I mentioned formerly, I tried a lot of other thing before asking.
Of course, I tried your suggestion as well! Unfortunately it didn't work.
I went further, and I installed the AC13 on a borrowed PC with hungarian Windows 7, and it works!
So, on one hand, you are right, the problem probably roots in the language collision, but on the other hand, I still can't solve the original problem: using AC13 on our own computer, with english XP, as we used the ac10, ac11, ac12 in the past years.
The answer to your other question: yes, it's a legal copy! All of the softwares, including OS are legal on our PC!
The AC13, as I also mentioned in my first post, is an education licensed version, which I downloaded from myarchicad.com, and installed with a unique serial number, which belongs to my wife, who is an architect student working on her diploma. Her partner in the student program is CAD Studio (Panyi Peter).
Regards,
pepicsek