I'm sorry to hear about your father.
It is possible to reset the admin password to a Mac quite easily providing you have the correct install disc to match the version of the software that is running on the computer. Resetting the password does not delete any of the information on the Mac, but it will prevent you from gaining access to other passwords that are saved in the system for things like mail accounts or wifi keys that have previously been entered and saved by the original user.
Once you have changed the password you will be prompted to create a new Keychain, which is the name of the place where these other passwords are saved.
Once you have admin access you can create or delete accounts.
There is a support article on the Apple website with info on how to reset the password here: http://support.apple.com/kb/HT1274
If you don't have the install disc or need more help, I'd suggest booking an appointment at the Genius Bar at your local Apple Store and they will help you reset it at no charge.