Why can't you use the CD ROM drive?
Which OS? Which make and model of computer?
As I understand it, it might be possible to install an OS from a USB drive, but if the computer has a BIOS then the BIOS needs to be able to see the external drive as bootable, and needs to be top of the boot order.
Maybe an external CD Drive?
How can I install an OS to my pc without using DVD ROM.