I think most here have the opinion that a used Mac, with the password left on it, and no disks, is very suspicious. You don't know what the last owner may have left on it.
Do you know the previous owner? If so, ask for the set of restore disks, so you can erase the hard drive, and get rid of everything that might have been left on the hard drive. Then you can restore the original software. It's very simple to do.
If you purchased this used at a shop, maybe they know how to get you some replacement disks....
Apple might provide you with replacement restore disks for a nominal fee, or you could just purchase an OS X install DVD. and start from fresh with the new Leopard.