Having problem with reseting my administrator's password

Thanks for the response, but.

It does nothing with the installation disk and/or a bootable USB. It just show the apple and does nothing.

Last night I try the target disk option and erase all my hard drive using my macbook and also installed the OS X the same way. It show that the installation was successful because it did ask me my name, new password, photo, administers password and all.

But when disconnecting the firewire and restarting the iMac, again it got stuck on the apple.

Please help.

Reset the SMC, here's how:http://support.apple.com/kb/ht3964

Next reset the PRAM and NVRAM: http://support.apple.com/kb/ht1379

Next try staring up in Safe Mode by holding down the Shift key on start up. Please let us know your results

Ok, first I did a reset for the SMC and then for the PRAM and NVRAM but the iMac did NOT start in any mode, safe, from the install disc, nor from the USB. Only in target mode from my Macbook. So this is what I did next:

Just finish erasing the hard drive using target disk with firewire and disk utilities. And installed OS X also using target disk and my Macbook, and it installs fine I can navigate the entire OS but ONLY from my Macbook using the firewire cable.

Now it will NOT boot by itself, nor using the install disc.

So I'm even deeper with my problem.

Please any suggestions??

If you're using a gray machine specific disk for the MacBook, that system will not work on the iMac. Try using the install disk that came with the iMac or a retail version of 10.6. Do you have a retail 10.6 system installation disk?Have you changed the hard drive on the iMac?

The first thing I used was the installation disk with the 10.6 system and it wont recognizes it.

When I used the "C" key option when restarted the iMac it won't do anything, when using the "option key it does give me the choice of what disk to use for start, but again it does nothing. So I used a USB and copy and mounted the installation disk to it, with the same results.

So I used my macbook and set up the iMac into target mode and then erase the hard drive (Zero Out) and then install a clean version of OS X using the USB. Still on my macbook, using the hard drive from my iMac I can browse everything, so I restarted the iMac using Start disk option on it and selecting the iMac's own hard drive, SAME RESULT.

Again using "option" key it does give me the choice of a start up disk but it wont boot up unless I select my macbook own hard drive and on my macbook selecting the iMac's hard rive.

Again thanks for the responses and I hope you can still help me because I'm loosing my mind. Thanks.