Password recovery / Other ways of logging into old iMac

I have an iMac, not sure how old but seems over a decade old, that my boss has had me trying to log into. I have tried almost everything on the internet and nothing has worked. We don't have an apple drive disc and my boss doesn't want her data deleted...Is there anyway that we can recover a password of her deceased sisters iMac? Is it possible for me to use her Apple account or mine to log-in? The iMac is very security tight and basically wont allow me to do anything, wont allow me to create a new account or guest account, keeps asking for password. If anyone could please assist me or refer me to someone who could assist me that would be greatly appreciated. I believe software is Snow Leopard