If I understand correctly, you're unable to boot the installation DVD because your firmware is launching Ubuntu directly. (This is a little unclear because you've got too many instances of "it" in your first paragraph, making it hard to disentangle their intended referents.) If so, the solution is in your firmware. Chances are hitting a function key as the computer powers up will bring up a boot manager menu that will enable you to boot the DVD. Unfortunately, the details of what key to press vary from one computer to another. The time window for pressing the key is also likely to be quite narrow. Repeatedly pressing the key from the time you hit the power switch is the best way to get in on most computers.
That said, instead of re-installing, I recommend launching to the Ubuntu live CD/DVD and launching GParted. This will enable you to re-size your partitions, which should enable you to continue using your current setup. (You may need to deactivate swap space before you can move or resize that, and sometimes other, partitions.) That said, there's always a risk of data loss when re-sizing partitions, so be sure to back up any critical data before you begin.