Lost admin password for apple mac os x 10.5.2

sounds like your hard drive has come unplugged, you will have to remove the battery and possibly the "L" shaped metal strip then you can push the hard drive back in

From Apple's Website.
OS Changing or resetting an account password Snow Leopard and earlier

From Wiki How (with screenshots)
How to Reset Lost Admin Password on Mac OS

Creating a new Admin on Mac Os X:
Here's how to reset your OS X password without an OS X CD.
the Working solution for me was to create a new admin
you can create new admin like this by deleting a specific file.

You need to enter terminal and create a new admin account:

1. Reboot
2. Hold apple key + s key down after you hear the chime.
(command + s keys on newer Macs)
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 your good to go again!

If you know the current administrator password, you can change it by going to System Preferences: Accounts.
If you've lost the password, you need Disk 1 of the OS install disk set. Start up the computer and put the disk in the optical drive. Restart the computer and hold down the option key immediately after the chime until a disk icon appears on the display. Wait a few seconds more until the optical disk icon appears. Click on this, then on the arrow to boot up from the install disk.
When the OS installer appears, go to the Reset Password... item in the Apple menu and follow the directions there. (Select the administrator account from the pull-down menu.) If your install disk is not specifically for an eMac, or is made for an eMac with a different graphics chip, the display quality may be poor, but you should still be able to pick out what you need to see from the Password applet.

Creating a new Admin on Mac Os X:
Here's how to reset your OS X password without an OS X CD.
the Working solution for me was to create a new admin
you can create new admin like this by deleting a specific file.

You need to enter terminal and create a new admin account:

1. Reboot
2. Hold apple key + s key down after you hear the chime.
(command + s keys on newer Macs)
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 your good to go again!

Restart the computer with the original Mac OS Disk hold down the C Key and restart the computer
Choose your Language
Hit Continue
Go to the Mac OS install page
At the top of the page
Click on Utilities
Reset Password
reset your password twice
Quit Reset Password from Apple Menu
Quit Mac OS Installer
When it asks are you sure
Click RESTART
You have now reset your Admin Password

If you have your Install CD
Start up from a Mac OS X Install CD.
Hold the C key as the computer starts to boot from the CD.
When the installer starts, choose your language, and you'll proceed to the welcome dialog.
Choose Reset Password from the Installer menu
Select your Mac OS X hard disk volume.
Select the user name of your original administrator account.
Important: Do not select "System Administrator (root)". This is actually a reference to the root user. Do not confuse it with a normal administrator account.
Enter your new password. Click Save.
Quit the Reset Password tool, and Restart your Mac.
Once you restart your Mac, you should be able to log in with the newly changed password.
If you don't...
This should work for your version:
To reset your OS X password without an OS X CD you need to enter terminal and create a new admin account:
Reboot
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
After rebooting you should have a brand new admin account. When you login as the new admin you can just delete the old one.

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Try this :-
Creating a new Admin on Mac Os X: Here's how to reset your OS X password without an OS X CD.
1. Reboot 2. Hold apple key + s key down after you hear the chime. (command + s on newer Macs) 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 your good to go again!
Apple stores wont reset it for you. Computer shops may have charged you $50 to $200 trying to reinstall the Mac.

It seems that while installing Mac, you selected a password for admin, and so, before installing any software you should insert the password.
If your Mac has not any admin password, I suggest you that make an admin password and now check to install your software by your new password.

from apple:"From page 31 of the "Welcome to Mac OS X" booklet:
If you forget your password for your computer, insert the Mac OS X CD and restart your computer while holding down the C key on the keyboard. When the Installer appears, choose Reset Password from the Installer menu and follow the onscreen instructions.
Tip: Resetting an account password doesn't affect other kinds of passwords that might be forgotten, such as your email account password, .Mac password, secure server , Internet access password (if you have one), Keychain passwords, or Mac OS X 10.3 Panther's master password. "

If that does not work, follow the indications at the bottom of this page: Macintosh OS: Lost Admin Password