So called 'live' distributions of Linux are very cool because they provide you a completely functional operating system without touching the existing one on your hard drive (usually Windows). It's like a one night stand: You have your fun, and when your done, you go back to your normal life without any commitment and without anyone being any wiser. They are absolutely vital for doing things like forensics, security mishap recovery, and crash recovery, because they completely bypass the existing software security measures and allow you unrestricted access to the hardware on the computer. There's many 'live' distributions of Linux which you can download, burn to a CD, boot off of, and when your finished, take the CD out and reboot back into windows like nothing happened. Knoppix just happens to be a more popular 'general distrubution' targeted at desktop Linux users.
Password protected BIOS's however do NOT provide any security. If the person is able to drop a CD-ROM in your computer and hit the reset button, they are equally capable of shorting the BIOS Reset jumper on your motherboard. It also won't stop them from taking your hard drive out and sticking it in a non-BIOS-password-protected computer.
One of the rules of computer security is 'physcial' security. If the person in question has physical access to your computer, your computer is not secure from that person. Period.
Things like encryption can keep the person from reading your personal files, but it won't stop them from taking a large mallet to the hard drive. Denial of Service attacks (ie turning the machine off) is a real threat in terms of computer security.
Physcial security should not be dismissed with trivial solutions like BIOS passwords.