1. Your guess is right - C is the XP boot partition.
2. Well, PM has done this several years without any problem. But you are right, I should have been more cautious.
I tried to repair this problem by booting form a PM Boot CD but PM can't read the partition (though the NTFS is recognized and the Size is recognized as well).
Right now I am saving my files using linux - this works smoothly. My next idea is to boot from XP-CD and to try chkdsk /f. My guess is a bad MBR and maybe XP can repair it.
It would louse uo my next days if I had to reinstall XP and all my applications from scratch. That's why I ask the community