I have written about creating bootable USB disk using free software, but it is of no use if the motherboard or BIOS does not support USB booting, if you are stuck in a situation where BIOS does not support booting from USB or CD-ROM, free boot-manager PLoP would be the best solution for the problem, the easy to use boot manager can work with your existing boot-loader and even without the need to install on hard-disk.

PLoP Boot Manager can launch from floppy, CD, network or existing boot manager like LILO, GRUB, Windows boot menu and DOS, once launched you can use PLoP to boot from USB and CD-ROM even if BIOS does not support these options.