try booting you machine is single user mode
Then edit the /etc/passwd file to remove the password entry of root ( make it null )
then the password shall be reset to just an enter key
then reset the password to what u want.
Another way of doing it is to boot from the CD,
mount the file system that has your root under /mnt directory and then edit the password file.

hope it helps.
what unix are u using ?

Milind.