The existing administrator password (and other user login information) for Windows is only valid in the original instance, and cannot be used on the new "hardware" that you're starting the AMI on (despite the fact that it's completely virtualized).
RDP connections to newly created instances, as well as efforts to get the administrator password, will fail because of this. Unfortunately, you won't be able to do anything with the new instances you've been trying to connect to, so you'll have no choice but to shut them down.
You appear to have completed all of the steps required to get this running except one: you never took any measures to assign a new password to your newly-launched instances based on the original AMI you prepared.
To fix this, run the Ec2ConfigService Settings tool (found in the start menu when remoted into the original instance using RDP) in the original instance and enable the option to generate a new password on next reboot BEFORE turning your instance into a custom AMI that can be used to launch new instances.