"This will not protect your system against boot from a LiveCD:
To do this, edit the BIOS to remove the CD/DVD drive from the boot
options, then password-protect it.
"Remember that if you forget this password, you won't be able to
re-edit the BIOS if you do need to boot from a CD! Finally, bear in
mind that a serious attacker will not be thwarted by these
measures. If you need more protection, look at your site's overall
physical security, and consider encrypting your hard drives."