If you forget your password for root, then you will be in a spot of bother. One way to fix this is to reboot the machine using the Debian installation floppy or CD. When you get the first question (about the having a colour monitor) press CTRL-ALT-F1 to get a shell. Mount your root partition (let's say it is /dev/hda1):
# mount /dev/hda1 /mnt
then edit the password file to erase the encrypted password for root:
# editor /mnt/etc/passwd
Save the file then reboot (CTRL-ATL-DEL).