How to Reset OS X Password without installation CD

To start the process you need to enter terminal and create a new admin account:

1. Reboot.
2. Hold Apple + S down after you hear the chime.
When you get text prompt enter in these terminal commands to create a brand new admin account (hitting return after each line):
mount -uw /
rm /var/db/.AppleSetupDone
shutdown -h now

3. After rebooting you should have a brand new admin account. When you login as the new admin you can simply delete the old one and you’re good to go again!

Source:Gigaom.com