I have forgotten the root password

Thanks for the very informative reply, but no, it was not the admin password. In an attempt to change the short name of the admin account, I logged in as root and changed user account information, then when I attempted to disable root user log in, somehow I messed up the root account password and I really don't know what is the password now. I figure I can wipe the hard disk and reinstall a new system, but is there an easier way to just reset the password?

Thanks.

No, I have came to the password window but whatever I put in, it says it's incorrect. I cannot remember exactly what caused this, seems that instead of doing a plain authorization, I had opened a change password window and immediately input the previous password then realised that it was not the correct window and immediately hit canel but perhaps ok was actually hit. I think eventually I might have to find some way to backup everything and wipe out the harddisk and reinstall.

I didn't know there was such an option, thanks for the information, 10.4 is fine because that's the highest OSX the PowerBook will run. Short of a solution, I have borrowed a hard disk for backup and reinstalled the OS, now there's another problem -- I cannot install OS9.2.1 using the retail installer disk and I cannot find my original restore disks! But thanks for the help anyway.

Thanks for the very helpful hints, unfortunately mine is the even older 550MHz G4 and Apple stated that the highest OS it can run is Tiger 10.4.11.

This whole series of trouble begins when a friend of mine lost his laptop, and I realised I should make access to my although old yet still quite usable PowerBook a little more difficult, first I experiment with log-ins and passwords to make it less easy to guess, then I intend to set the FirmWare password which is not possible with my old firmware installed. According to Apple, firmware updates have to run under OS9 which I haven't used for a long time. You know most of the rest of the story. My retail 9.2.1 installer came together with 10.4 can boot up the PowerBook, but when tried to install, it said it cannot be installed in this computer. I also have retail OS9.1 installer disk, but that won't boot the PowerBook. May be this is a good thing because the procedure of setting up FirmWare password seems a bit complicated to me, may be I shouldn't mess with that!

I didn't even know that! Thanks. I'll take your advice.