If you forget your password then this Mac 10.6 reset password post is for you. This post also assumes you can't boot from the 10.6 install disk (or USB).
Reboot the computer and hold ‘⌘ (Command) + S' while the computer boots. This will enter single user mode. When the command prompt appears enter the following commands:
mount -rw /
shutdown -r now
Once you are back up and running, kick off a Permissions Fix via the Disk Utility.
What This Procedure is Doing
What you are doing is booting into single user mode, mounting the root volume with write permissions, running the normal startup script to get the system up and running (stay in single user mode and not start the GUI), change password, and shutdown and reboot.
Single user mode shows the full details of your Mac computer's boot process. Right before it would normally kick off the GUI, it allows you to enter commands at a terminal screen signified by root#. If you're familiar with Linux, you seem some similarities.
If you get stuck with a big lock icon and a password prompt, you're looking at a firmware password challenge. You'll either need this password or the install disc (or USB). To boot from the install media, hold the Option key while booting so you can select it.
Firmware passwords lock you out of the computer so if you forget it, you'll need to contact Apple for assistance in unlocking it.
This has always worked for me, although I performed these password resets on single user Mac laptops. This Mac 10.6 reset password (Snow Leopard) discussion from Apple Communities shows a slightly different procedure. Let me know what worked for you.