First it makes a difference if we're talking about an EFI password or an OS password.
If you hold down the option key while booting, and you get a password prompt at a gray screen, that's an EFI password. You bypass that by removing one RAM module and then resetting the PRAM 2X in a row. If the laptop only has one RAM module, you'll need to get a second one. After resetting the PRAM 2X with an altered amount of memory, you should be able to get to the boot picker, and boot off a DVD or what have you.
If it's an OS password, that can be reset by booting off the OS restore DVD, then once you select a language in the Utilities menu there's a password reset option, which should be pretty self-explanatory. You can use any OS restore disc that will boot on that unit, not sure if a 10.5 disc will cut it, but just ignore the message saying how you can't install this version of OS X on this computer. You should still be able to access the Utilities menu.
My deceased mother-in-law (bless her) bequeathed me her Macbook 7.1 but not the password! It is running 10.6.4. In order to install new software I know I will need my own Admin password.
I tried booting to a licensed CD for OS 10.5 from my Imac, but the former Admin password was required. I guess there are two questions:
1. how to circumvent the lost password with a newly installed OS?
2. whether the operating system from an Imac will even work on the Macbook if I can get around the password problem?