The boot priority can be set in the BIOS. Have you ever worked with the BIOS before? There should be a prompt when the machine is powered on that says "press this key for setup." I think the key is F2 on your model. Turn on the power and press F2 as soon as the first black-and-white screen comes on.
This will let you select which device to boot from - disk, USB key, DVD. If you mean to install two OSes on the disk and boot to one or the other, the boot menu is at a higher level. Where exactly are you going to run Ubuntu from? I run my linux environment in memory loaded from a fast USB key.