.....I don't understand why you wouldn't be able to....
Then just insert the disk etc?
Like the guys already said above, you just need to select the external CD drive as the first boot device. Here you have instructions on how to do so for Phoenix BIOS.
He's asking if he can use an external DVD drive to install an OS, not about not wanting to install an OS.hmm.. couldn't understand why you don't want an OS on your laptop.
Cheap OS might not be as good as the "expensive" one.
Take a second thought.