another option, for the sake of anyone else looking at this thread, is to run a program like VirtualBox to emulate the other operating systems. However, this requires more processor speed and RAM but it is still possible and you don't have to worry about reconfiguring GRUB. Here is a link
The answer lies in BIOS. Enter BIOS (F2 at start up for me) and change the boot order to an "advanced" boot. This over-rides the Ubuntu start-up. Then, delete all the partitions and install whatever! :-)
Your advice re Ubuntu "uninstallation" is highly appreciated.
I just had an idea: Suppose I wanted to keep Ubuntu and Windows loaded in the same PC (120 Gb of hard disc). Would that be possible? And how?
P.S. Your English is great.