If an attacker has unrestricted physical access to a system, then, unless the data is encrypted, it becomes almost impossible to prevent full compromise. Programs such as the Ophcrack liveCD allow Windows passwords to be easily broken; while any liveCD will allow access to the data stored on the drives. It also becomes possible for an attacker to install malicious hardware, such as a keylogger, or a hidden wireless access point, allowing them into the network from outside, Because of this, it is crucial that you keep your site physically secure.