When your Ubuntu system boots, you will see the GRUB menu if you hit
the Esc key, or if you've enabled the
menu to show by default. The only issue with this is that the default
timeout is only 3 seconds. You may want to
increase this amount… or you may even want to decrease it.
Open up the /boot/grub/menu.lst file in your favorite text editor
sudo gedit /boot/grub/menu.lst
Now find the section that looks like this:
## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the
# (normally the first entry defined).
The timeout value is in seconds. Save the file, and when you reboot
you will have that many seconds to choose the menu item you want.