Q: complications resolving forgotten administrator password...

Hi. I have an imac 10.5.8 which been in storage for a year or so. Unfortunately I have forgotten the administrator password. I have spent most of the day in forums but am still unable to resolve the issue. I would like to either use terminal to set up a new administrator account, or preferably to use the startup disc to reset my password.

I am not able to reboot in single user mode. I must have set up firmware protection when i got the imac years ago. (I cant fix this without the administrator password.)

...so I can't open terminal or the start up disc from the login screen.

I have tried opening terminal from the guest account (the only account I can get into)  and using these instructions: 

https://discussions.apple.com/thread/3506318

but terminal tells me that I do not have permission to do this.

I have tried numerous times to open the reset password application in the Utilities folder in the startup disc from within the guest account, but everytime it quits unexpectedly.

At a loss as to what to do. Would appreciate any help!

Thanks.