By default, the EC2config service will attempt to set your computer name for you.  You can change this behavior by editing the Config.xml file inside your instance to have the element <Ec2SetComputerName> changed to Disabled.  More information on this can be found here:

http://docs.amazonwebservices.com/AWSEC2/latest/UserGuide/appendix-windows-config.html 

Thanks
E.Ravi