In a mobile computer equipped with Wi-Fi, you can (and should) password protect operating systems such as Windows XP. This makes it a great deal more difficult (although not always impossible) to boot up your computer without knowing the password.
You can also set a password in the BIOS of most computers. This provides a better level of security than an operating system password, but it is also far from absolute.
For technical information about the security level provided by BIOS passwords, and what is involved in cracking them, see http://www.heise.de/ctenglish/98/08/194/. The article also tells how the author used BIOS password protection as a disciplinary measure with his 10-year-old daughter (“We used to give ...