Problem in single-user mode

Hi everyone. I just purchased 5 different Imacs from a liquidation auction. I have been trying to reset the administrator passwords, but am running into a little problem. The reset worked fine via single user mode on 2 of the Imacs which are mid 2008 models with 10.5 installed, there was no problem.. However, when I boot up in single user mode on the newer imacs (2009-2010) with 10.6 installed, I am unable to type anything into the command prompt. I am using a Dell USB keyboard, but I know th ekeyboard is recognized by the system because holding down the WINDOWS + S button causes single user mode to start, but one it comes time to input text, it is as if the keyboard was no longer there... Anyone have any ideas f why this could be happening and perhaps how to get around it?

Thanks for your help, I have searched the site and found 2 or 3 posts on this subject but nobody has posted what they did to correct the problem.

Thanks you again for your help