First try something simple. Log in as normal user, then start Kuser. You'll have to type root's password. If you are lucky and can start Kuser, then clear the 'Account disabled' check box in the root account.
If you are using GRUB as a boot loader, you could try booting into run level 1 (single user mode). Manually add 'init 1' to the boot options. If you can log in at that point, type 'passwd' to change the root password.
If you are using Lilo then proceed with Willy's solution. When you boot with the installation CD/DVD, don't run setup. Just log in. Then type:
mount /dev/hda(x) /hd [You have to know the partition number in place of (x)]
With the installation CD/DVD you have to use vi as a text editor, so be sure to know the basics of vi (:w, :q, insert, Esc).
After mounting the root partition, type:
Somewhere in /etc/passwd, usually as the very first line, you should see something like this:
Notice the colons, which is why you have to understand the colon syntax with vi. You have to be in text insert mode before typing the colons, otherwise vi interprets the colon as a command.
Also notice that no password is present in /etc/passwd. That is because the password is stored in /etc/shadow. But you need an entry in /etc/passwd to enable the root account. Restoring that entry might be all you need.
If you need to edit /etc/shadow, here is a nice introduction to that file: Understanding /etc/shadow file
. You might need to delete the encrypted password before trying to run the passwd command.
Yes, there is a passwd command as well as a passwd text file in /etc.