Hello - good news!
After having read tons of articles in the Internet, laptop service manuals, centOS documentation, etc I was still "in darkness" with this. After having tried 10 different tricks of booting and inventing 50 new ones I was still completely lost. I also happened to read an article about order of the events that take place when booting....
...tick .. tack .. tick..... an idea came, which at that moment felt quite desperate.
I connected the power cable on and took the battery out: my sweethart runs as if there had never been anything wrong!
Why doesn't my laptop like the battery any longer? The battery is quite new and there weren't any problems before I installed the dual boot. Could somebody explain, how should I understand this?
Nevertheless, I am relieved - a moment ago things looked really bad! The lesson is, that it is aways worth the effort to post questions to this forum
((ADD: Referring to my original question there was no battery in, when I thought that using XP installation dvd had worked.))