Hi there,
This can get technincal, but there essentially 6 phases in the StartUp process of a computer. The first two are System Initialization and System Setup which are system independent, where you go into the BIOS and set the HDD boot order, enabled hard drivers, config the settings etc.
Then there is a phase called Boot Manager. If you have just 1 OS, you don't have a choice, if you have 2 or more OS in different partitions or different physical drives, you can have a choice where a Boot Menu is presented 15-30 seconds and you choose which OS you want to boot up.
Next comes the Boot Loader, where the OS you chose loads into RAM and begins.
So you can alter the Boot Manager Phase to choose the OS's you have available, and in which order you would like them to be. The first in order is the default one, if you do nothing, that one gets chosen.
You can edit the MBR and BCD yourself to make this work (very advanced) or you can have a small appl intercept this phase and let you set it up the way you want, from a GUI shell. Two programs that do this are free, EasyBCD and VistaBootPro. Usually you click on the Space Bar or F2 to bring up he Boot Menu just after the Splash Screen closes.
Take a look at Easy BCD, and give it a try. It scans your physical disks, lists the OS's you have, and gives you a Boot Menu choice to click one the one you want. This is called MultiBooting, or with just two, DualBooting.
When you choose, say Ubuntu, Vista doesn't open or interfere. When you shut down your computer, next time you boot up, you get the choice of which OS to boot into.
I think this is what you are looking for, and hope it helps