To break into a machine where you don't know root's password,
  - Boot machine
  - Interrupt grup prompt (presuming you have time) by hitting enter,
  - Enter "e" to edit the boot-time parameters,
  - Move down to the kernel line and type "e" to edit it,
  - append an "s" to the end of that line to enter "single user mode"
  - hit enter to save that line,
  - type "b" to boot
  - After booting, enter df to see what /dev/xxx to use
  - type "mount /dev/hda3 (or /dev/sda1) -o rw,remount /
  - Now you can fix the system or use passwd to change root's password.passwd

There's another way to do this by booting an install CD, but this is easier
and I would think, more common.