You have a server that is running Windows 2003 Server which you need to format and reload.
You are able to get the CD-Key out of the registry easy using a free program such as produkey (http://www.nirsoft.net/utils/product_cd_key_viewer.html).
How do you know what version of the windows media uses? There are five channels that you can obtain a copy of Microsoft Windows Server 2003:
- Volume licensing programs
- Checked build
The following Microsoft Knowledge Base article shows how to determine the windows flavour using a variety of methods. I use the Product ID method.