If you are running a Windows Server 2003 AMI it is most likely S3-backed. If you'd like to migrate it to an EBS-backed AMI so that you can take advantage of new features such as the ability to stop it and then restart it later, I've got some good news for you.
We've just put together a set of step-by-step instructions for converting an S3-backed Windows AMI to an EBS-backed AMI.
You will need to launch the existing AMI, create an EBS volume, and copy some data back and forth. It is pretty straightforward and you need only be able to count to 11 (in decimal) to succeed.
The document also includes information on the best way to resize an EBS-backed Windows instance and outlines some conversion approaches that may appear promising but are actually dead-ends.