remove preinstalled other OS from grub after Ubuntu installation

Warning: it can crash your system.
Be quite sure before doing this.

$ sudo vi /boot/grub/menu.lst

comment by '#' symbol section entries related to other OS.

you can also remove it from filestystem:

cd /media/sda1/
ls

it should give you info about folders of other os.

sudo rm -rf *

removes all data stored on the file system where other OS files were.

sudo reboot now

will reboot you to system clean from other OS.