Currently Being ModeratedJan 8, 2013 2:18 PM (in response to Keithdk)
Put your install DVD into the optical drive (CD/DVD drive) and reboot. Be sure to either use the disc that came with your Mac, or, if you installed a later Mac OS X version from disc, use the newer disc. As soon as you hear the boot chime, hold down the "c" key on your keyboard (or the Option Key until the Install Disk shows up) until the apple shows up. That will force your MacBook to boot from the install DVD in the optical drive.
When it does start up, you'll see a panel asking you to choose your language. Choose your language and press the Return key on your keyboard once. It will then present you with an Installation window. Completely ignore this window and click on Utilities in the top menu and scroll down to Reset Password. After resetting the password use Startup Disk from the same menu to choose your hard drive for startup and restart.
If you're running 10.7 Lion check this article
Currently Being ModeratedJan 8, 2013 2:23 PM (in response to frederic1943)
This is embarrassing, but what if I don't have the old DVD's?
Currently Being ModeratedJan 8, 2013 2:52 PM (in response to Keithdk)
If you don't have your install disc handy, then follow these steps
1. Start up in single-user mode (hold Command-S as the computer starts up).
2. Type: mount -uw /
3. Press Return.
4. Type this: passwd (user's short name) Replace "(user's short name)" with the short name of the user account whose password you wish to reset. If you are not sure what the account name is, you can get a list of user names by typing: ls /Users
5. Press Return.
6. Enter a new user password.
7. Press Return.
8. Type: reboot
9. Press Return.