You need to use the administrative password to add your own account as an administrator. Catch 22!
If you can't find that, you can always reload or repair install the operating system, and make yourself master of the machine.
However before you try anything, backup all your files to a DVD or thumb drive, etc...
you can try taking the battery out and reset the CMOS, don't know if that trick works for you, but it may clear the passwords...
when the log on appears, just hit enter instead of entering a password.