If you lost your system root password, it is not possible to change it without direct access to the machine.

You will need to boot the other Linux from the live CD, for example:

 http://bouncer.gentoo.org/fetch/gentoo-2007.0-minimal/x86/

When it boots, mount the root partition of the system from the disk
and edit 
/ect/shadow

place the empty password for the root user then reboot the system (eject CD) and you will be able to login as the root user without a password and set the new password.