Gathering local administrator accounts from desktops on network

Do you mean what accounts are local administrators? There is no programmatic way to get the passwords.

There are lots of article around on how to get the information. Here is an example: http://powershell.org/wp/2013/04/02/get-local-admin-group-members-in-a-new-old-way-3/

you could use computer GPO to starup script to run a command net localgroup administrators
the response will be the users/groups that are part of this group
you can have this response echoed into a file on a share that every system can write into.

you can script psexec to connect to each system and run this command. there are vbscript/powershell examples to do the same meaning connect to each workstation and enumerate groups or users........  As brian covered...

The other option is to use the AD GPO computer configuration and restricted group policy setting forth which users might be members of the builtin\Administrators restricted group will kick every single user that is not in the restricted group policy out...

You could also use a COMPUTER GPO to push out a local admin user creation/password setting to make sure you have a common local administrative account.

More details, please. Would you like to check password strength or why would you try to gather those?

We are in the process of cleaning up any non used local admin for each desktop. its a vulnerability prevention