How do you boot from OS X disk?

[MacGiver]

Hi, could you please help and tll me what key should i keep pressed to make sure my MBP boot up from the OS disk when i restart with the disc in....i want to reinstall idvd (which is acting up)...
Thx
X

[Eidorian]

Hold down Option or C after the boot tone.

[MacGiver]

thx...you are everywhere? :-)

[Eidorian]

thx...you are everywhere? :-)I try.