You're using Snow Leopard according to your computer information. This is the Tiger system forum. Still, maybe these still work:
[Changing or resetting an account password|http://docs.info.apple.com/article.html?artnum=106156]
[Changing your administrator password|http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh655.html]
[Mac OS X: Changing or resetting an account password|http://support.apple.com/kb/HT1274] - resetting password on regular and administrator account if you have the installer disc.
Here's one method to reset the password without the install disc:
1. boot computer and hold the "apple" key and the "s" key.
2. wait for terminal show
3. release keys
4. type without the quotes: "/sbin/mount -uaw"
5. press enter
6. type without the quotes: "rm /var/db/.applesetupdone
7. press enter
8. type without the quotes: "reboot"
9. press enter
This will let you create a new admin account when you reboot your computer.
Also, without the discs, look at Mac OS X Tip - No Password, No Root, No CD? No Problem.
Here is a way to get around not knowing the admin password on Macs.
To reset your OS X password without an OS X CD you need to enter terminal and create a new admin account:
2. Hold apple + s down after you hear the chime.
3. 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
4. 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!