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 not absolute.
To set a BIOS password, you must enter the BIOS screens for your computer. This is done during the boot-up process when you've turned the computer on, generally by pressing a key (such as the Delete key) or key combination while the computer is booting up.
For technical information about the security level provided ...