Simplest solution: Boot into the Ubuntu, start a terminal (should be in the menu under Accessories or similar) and type
sudo dd if=/dev/zero of=/dev/sda bs=1M count=1
You will be asked for your password, and you have to type it blind, so type carefully.Do this only, if the Ubuntu harddisk is the only one in your system.
If you have more the one disk, you have to check which one is the one with the Ubuntu on it. This can be done with
in a terminal. Then replace the sda
in the command above with the one you found Ubunt on.
If you have done that, reboot and install Windows.