It depends on a computer or OS versions
you may try:
Add or remove a stick of RAM (change the amount of RAM - if you have one stick in, add one and if you have two in remove one).
Power on the Mac and immediately press and hold command-option-P-R.
The system will restart with the ‘bong noise’, allow it to do this 3 times. On the third ‘bong’ you can let go of the keys.
The machine will now boot with a cleared password and reset PRAM/NVRAM.
You can shut down the machine and replace the original RAM configuration.
For later computers if you do not know the password the only option might be to shedule a meeting with Apple Genius....
there is another way, which is far too complicated http://jerseycityrepair.com/remove-firmware-password-macbook-air/
I am not sure that Snow Leopard disk will solve your problem, as the reset password utility may require an original password to change it.