Physical+security

If you are a home user, you probably don't need a lot (although you might need to protect your machine from tampering by children or annoying relatives). If you are in a lab or work place, you need considerably more, but users will still need to be able to get work done on the machines. If you are in an office, you may or may not need to secure your machine off-hours or while you are away. At some companies, leaving your console unsecured is a termination offense. Examples of physical security:


 * Locks** - Many modern PC cases include a "locking" feature. Usually this will be a socket on the front of the case that allows you to turn an included key to a locked or unlocked position. Case locks can help prevent someone from stealing your PC, or opening up the case and directly manipulating/stealing your hardware. They can also sometimes prevent someone from rebooting your computer from their own floppy or other hardware.

Many PC BIOSs let you set a boot password. This doesn't provide all that much security (the BIOS can be reset, or removed if someone can get into the case), but might be a good deterrent (i.e. it will take time and leave traces of tampering). Similarly, on S/Linux, your EEPROM can be set to require a boot-up password. This might slow attackers down.
 * BIOS security** - The BIOS is the lowest level of software that configures or manipulates your x86-based hardware. LILO and other Linux boot methods access the BIOS to determine how to boot up your Linux machine. Other hardware that Linux runs on has similar software. You can use your BIOS to prevent attackers from rebooting your machine and manipulating your Linux system.

Obvious physical security methods such as locks on doors, cables, locked cabinets, and video surveillance are all good ideas.