What do you use as boot manager, is it grub or lilo.
First consider lilo...
1. When lilo shows the graphical screen, press Ctrl+x, and go to terminal mode.
2. Enter linux S, where linux is the label
3. You will be booted into single user mode now. Type passwd and type in your new root password.
Now, we shall see grub...
1. When grub shows the graphical boot, press the option to edit( I think it is 'e')
2. In the second line of the three lines, add "init 1" at the END.(At the end of the line, append init 1. If you add it somewhere else, the system won't boot)
3. You are booted into single user mode. Change the password.
That's all folks!