If it is a Multiple Users password, there is an easy (sort of) fix.
You will need to boot the machine to a volume (disk or drive) other than the normal hard drive. This other volume can be an external hard drive with an OS valid for that model, but is typically an install CD of appropriate version, such as one that came with the machine or a retail (white label) one of a valid OS version. A bootable utility CD, such as one for Tech Tool or Disk Warrior, is also suitable provided it uses an OS version valid for your machine.
Once you have the machine booted to this other volume/disk, open the System Folder on the hard drive. In there, locate and open the folder named Preferences. In that folder, locate the file named Multi-User Prefs. Throw that file away and empty the Trash.
Then do a normal restart, booting to the OS 9.1 on the hard drive. You should now get no request for a Multiple User password.
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The reason that you heed to boot the machine to a different volume is that you need to get it running using an OS other than the one requesting the password, so that the contents of the System Folder can be accessed.