Using the Ubuntu 12.04 Live CD, is possible not to install Grub? In previous versions, there was a checkbox called something like "skip grub installation", now in 12.04 this checkbox isn't there any more.
It has been many years since the bulk of this answer was the correct answer.
The solution now is to start Ubuntu from a live disc and use the terminal to invoke Ubiquity as follows:
ubiquity --no-bootloader
You can alternatively use the short form like this:
ubiquity -b
The following is no longer valid, but I will preserve it as it was originally when the question was asked and it is possible somebody may want to do this with Ubuntu <= 12.04
Installing GRUB is the usually the best way to go, whether or not you're dual-booting, but to install Ubuntu 12.04 without GRUB, download the alternate CD for x86 or AMD64.
Run the install as normal, after the Select and install software step, the installer will run Install the GRUB boot loader on a hard disk. It will tell you what other operating systems, if any that it's found, and ask if it should install, at this point, choose Go Back, then in the installer menu that appears next, choose Continue without boot loader.
Please be aware that after installation you will have to configure your existing boot loader before you can boot your new installation of Ubuntu.
