How to reformat and re-install an old G5
You can restart while holding down the C key to boot from a valid Mac OS X Install CD / DVD for that G5 based Mac. To pick and choose what disk your machine can boot from hold down the option key while starting the Mac.

Also depending on which version of Mac OS X is installed there are different ways to reset the password in single user mode. This may be an option for you if you just want to  unlock and add / remove users and you have a startup CD/DVD for that Mac.

See  Changing or resetting an account password for some instructions on how to reset the password.