how do i reset password on SNOW LEOPARD without the installer cd?

My parents can't remember their password (and/or username possibly?) to their desktop, whose OS is 10.6 (Snow Leopard). They haven't found the installer CD yet.

I know how to reset the password on Lion or later – super easy – 'cause I've done it successfully with my MacBook. I've found some methods online for Snow Leopard, but I'm a little nervous about trying them... I guess worst case scenario we can't access it and have to take it to the store for help, right...?

Thoughts?