Windows Server 2008 and Windows Server 2008 R2, based on the Microsoft Vista codebase, is the last 32-bit server operating system released by Microsoft. It has a number of versions, including including Foundation, Standard, Enterprise, Datacenter, Web, HPC Server, Itanium and Storage; new features included server core installation and Hyper-V.
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I'm not sure if this works in a RAID setup as well but you should be able to do it with system integrated tools.
Open the Server-Manager and expand the "Storage" option.
Go to disk management.
rightclick on (C:) and choose downsize (or similar meaning).
This should do it.
Regards