Thank you for all your help. But as I told you, I don't want to modify the code, as this is not allowed. I also saw the thread about this nice plugin, but as I said, I dont want to modify the code I get from Google.
I have been working with the built-in rich text editor. To paste the code, I use the HTML source icon. I have now even installed a better WYSIWYG editor, as I chose WordPress because of it's ease of use and not to do it all by hand in HTML. You get my idea.
It seems that the WYSIWYG editors ad some code even when working with the source HTML. And completely disabling the WYSIWYG editor is not the point of using WordPress.
By the way, this forum here, and the plugins for WordPress and even WordPress itself does not really work with Internet Explorer (newest version) nor Opera (newest version), but it all works flawlessly with my old Firefox 1.0. WordPress seems to be too Firefox oriented, at least from the editing side.