I finally tried to use a memory stick as my desktop system rather than a box of hardware. Now I have my full system on one little stick. That’s cool.
1) I downloaded the latest Ubuntu 9.04 ISO Image and saved it to the HD
2) With K3B I burned the ISO to a CD.
3) I rebooted and bootet the Ubuntu Installer from CD. I choosed the memory stick as the device whre to install the Ubuntu desktop.
4) Afte Installing, I rebooted again the new Ubuntu Desktop from memory stick
5) After the first boot, Ubuntu said, that the language packages aren’t complete, and offers a wizard to complete them, by installing the missing packages.
6) But I can’t download them, because I had no WLAN access. The whole Wireless part of the networkmanager was dead. I got a little bit in panic, but in the end I found out, that the Airport wireless antenna on my Mac Mini was full of dust. So after cleaning it manually 
It worked fine.
7) While downloading the missing language packages the package manager told me, that some repositories where not reachable, that’s not so good, but looks like I don’t need them at the moment. In the end I downloaded another 18 language packages.
8) Now that I allready downloaded the new repository files to the package manager, I also did a generell update of the system, as it was adviced by the update manager. 75 new and updated packages were downloaded and installed
10) That’s it. I finally got a new Ubuntu 9.04 desktop system that is great to use and easy to install. What else do I want?
ThorN