You problem is related to the boot order of the devices in the bios section of the laptop.
On the post screen of the laptop you have the option to enter the bios setup screen.
The keys to press can be F1,F2 or Del.
Once in the bios you need to select the boot option menu, there you should be able to select what device the laptop boots from first.
If you have the option to boot from Usb, since the external cd/dvd drive is most likely to be the case set it to boot first in the list.
Then save the settings in the bios. restart it should say press any key to boot from CD.