In case you have forgotten the root password, the following should do the magic:

  1. boot the system from the installation ISO
  2. upon reaching the network configuration part press the ESC key
  3. on the Debian installer main menu select "Execute a shell" and continue
  4. mount the partition by issuing mount -t ext3 /dev/sda1 /mnt (*)
  5. type chroot /mnt
  6. issue the passwd command and enter the new password twice
  7. type exit
  8. unmount the partition by issuing umount /mnt
  9. type reboot or telinit 6 to restart the system

*: depending on the format of the disk, there could be "ext4" instead of "ext3" also - if the mounting operations fails, this is the most probable reason