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