If one have a local access to the machine (meaning physically), the administrator account password can be changed by booting up a linux cd, flashdisk or floppy.
Even linux servers can be "compromised" using this way.
In a more secure environment, servers are on a locked room for added security.
Remote/networked PCs is another case. Compromising remote machines are done by using known "bugs" that causes buffer overflow. Code exploits are then pushed into the stack and the buggy programs are then "stack-overflowed" thus the exploit codes are run from the stack. Then the machine gets "compromised" and the password and security credentials can now be changed easily.