generally i believe if you delete the password for root in /etc/passwd you will be asked for a new password upon next reboot. this is the case correct? (luckily ive never lost my password
)
but you cant do that with the OS that is on there if you dont already have the root password.
so, you have a few ways to do it. your goal is to mount that file system and change the /etc/passwd file.
first thing, you can rip out that harddrive and put it in another computer with an os on it already, so you can then mount that harddrive and edit accordingly. that however is a pain in the ass.
so, the easiet thing to do would be to boot from some cd and use that to mount that drive and edit the file.
your install cd is its OWN os. if you boot from that disk, look for an option to get to a command line. once you have a command line you are set. mount that drive, browse to
/etc/passwd and fix your computer!
as perderabo said, search, youll still find more info than what i have provided.