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