Many people -- in particular hardware vendors -- are nowadays supplying bootable ISO's.
You have to
1. Download the ISO
2. Burn a CD
3. Boot with it
4. Follow the instructions (to (re)flash the motherboard/CP/Disk etc)
Ive already been able to boot ubuntu from an iso on disk
Next Ive done it with making a grub-bootable usb and loop-mounting the iso from there