1. Download pspasswd.exe (More info)
2. Create computers.txt file and place it in the same folder as pspasswd.exe
3. In computers.txt list all computers you want to reset the admin password. Each in a new line.
4. In the command prompt run pspasswd.exe @computers.txt administrator NewAdminPassword

Make sure you have domain admin rights.
All files including a simple .bat script can be downloaded from here.