AWS allows you to create an AMI image from an existing EBS snapshot. This process is called "Registering":
However, registering an AMI image from an EBS snapshot will only work for a Linux EC2 instance. It will not work for a Window EC2 instance.
If you attempt to register an EBS snapshot created from the root EBS volume of an EC2 instance, the AMI image that is created will say it is "Linux".
How to Create a Windows AMI Image
To create an AMI image for a Windows EC2 instance, you must create the AMI image directly from the EC2 instance.
Using the AWS Management Console
Using the AWS Management console:
Find the Windows EC2 instance in the list of EC2 instances.
From the "Actions" menu, select "Image" then "Create Image".
Complete the dialog that appears.
Creating AMI images from a Windows EC2 instance can be done using any of the following actions:
Create AMI Image - Creates an AMI image of one EC2 instance
Create AMI Images - Creates AMI images of one or more EC2 instances