The default password on the Windows Administrator account is set automatically on all instances to a random value by the ec2Config service. You can retrieve the password originally set by using the ec2-get-password command, as shown here:
C:>ec2-get-password i- -k gsg-keypair.txt
Alternatively, you may use the “Get Administrator Password” command in ElasticFox by right-clicking on the instance. The first time you use this feature in ElasticFox you will be asked to set your gsg-keypair. This is the keypair you generated when you first configured your AWS account. The path to the gsg-keypair file is stored in the ec2ui.last.ec2privatekey.file Firefox variable.
Error in ElasticFox: “Invalid EC2 Private Key”
Reference: Windows on Amazon EC2 Security Guide