Where Are Passwords Stored On a Computer

Login passwords are not stored in the computer. Only a hash of the password is stored and that in a location that is not normally accessible to even an administrator. There is nowhere in the computers hard drive you can see login passwords.

Your options are few:
Obtain a valid password from the owner or reinstall the OS (unless it is a BIOS password).

Never buy a used computer from anywhere unless you get all necessary passwords.