the version installed is ubuntu 8.10 i386 which I want to deinstall completely. So far I could not find a function in this version by which I could do that. Thanks a lot for a helpful advice.
The only operating system I know that has an uninstall feature, is Ubuntu when installed in a virtual filesystem inside Windows. If that is the case, then you can uninstall it from Windows' add/remove pane in the control panel.
There's normally no reason to uninstall an operating system. You can't use your computer without an operating system anyway, and all operating system installers are able to replace the one currently installed. If you want to delete everything on the harddisk, then you can simply format it. You can do that from an Ubuntu live CD/USB stick for instance.
If you want to replace 8.10 with a modern version of Ubuntu – and you really should – then it will be able to keep your data while replacing the operating system. It's quite easy.