Now this is really cool! Using ElasticFox, Amazon’s Open Source tool for managing machine instances on their EC2 service you can easily boot a Windows 2003 server instance.
1. Select an AMI (a machine configuration)...
 | 2. Select hardware configuration, security options then boot the machine...
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3. Wait a minute or two for the instance to boot then grab the Admin password and login via Remote Desktop:
Now, what to do next hmmm.... :-)
Btw, once you have an instance up and running you might as well keep it around for at least an hour since that’s how it’s billed. In this case a "small" instance without Authentication Services runs $0.125/hour.
[UPDATE Oct. 26 2008] Btw, here is your Getting Started Guide & Setting up an Account to EC2