This is on-top of the existing 750 micro-instance hours a month for linux instances! Which means you can now run 2 micro-instances of Windows and Linux each under the Free Usage Tier!
From AWS Blog
The AWS Free Usage Tier now allows you to run Microsoft Windows Server 2008 R2 on an EC2 t1.micro instance for up to 750 hours per month. This benefit is open to new AWS customers and to those who are already participating in the Free Usage Tier, and is available in all AWS Regions with the exception of GovCloud. This is an easy way for Windows users to start learning about and enjoying the benefits of cloud computing with AWS. The micro instances provide a small amount of consistent processing power and the ability to burst to a higher level of usage from time to time. You can use this instance to learn about Amazon EC2, support a development and test environment, build an AWS application, or host a web site (or all of the above). We've fine-tuned the micro instances to make them even better at running Microsoft Windows Server.