thank you all for your answers.
I've got XP. The error message (The German version of "rundll32.exe has found a problem and must be ended") often occurs just before an installation is about to complete, or, just now, as I was trying to turn a picture by 90? in Windows Picture Viewer.
The error message also occurred when I tried to install XP SP2.
I only have a Product Recovery Disc. Do I find "rundll32.exe" there, too? - If not, I'll try the dial-a-fix programme.
Thank you once again