I need to have my computer re boot FROM A DISC and NOT from the installed system.?

I have a fairly new (6 month old) Sony Vaio that came with windows 7 home premium. I upgraded to 7 ultimate (not exactly legally) and of course I'm having problems. I have a 7 home premium disc but every time I turn my computer on it goes right to "starting windows ultimate" I know there is a certain key (or combination of keys) to push right at startup that will give me the option to reboot from a disc but I can't seem to remember it. Can anyone help?