There’s a special key in every BIOS (when the computer first boots) that makes you able to boot from another device that is not its hard disk.
You can either enter in the bios to configure the boot order or to hit the key that selects the device to boot “only this time”, the second option is suggested.
In short, you simply need to turn ON your computer, then immediately press multiple times the F’ key that a message should tell you something like “boot menu” until a menu appears, then just select the device which you want to boot from (the one that contains Elive in your USB or DVD).
This special F’ key is normally F12 in Acer computers and ESC in the Asus ones, you can try all the F’ keys or better search which F is in a list like here, other common F’s are F10, F2, F1 and F8
Here is the list of function keys used by a manufacturer:
Acer – F2 or delete
Asus – F2, F9 or delete
Compaq – F10
Dell – F2
Emachines – tab or delete
HP – Escape, F1 or F10
Lenovo – F1 or F2
NEC – F2
Packard Bell – F1 or F2
Samsung – F2 or F10
Sharp – F2
Sony – F1, F2 or F3
Toshiba – Escape, F1, F2 or F12
If your computer uses those newer BIOS that ships with Windows 8 with the restriction to use other operating systems, you need to enter in your BIOS setup and basically: