You can use whatever operating system you see fit, whether it be another flavor of Windows or a free alternative like FreeNAS and any number of Linux distributions, but no desktop OSes at all, only server OSes. Necessary drivers are provided for:
Microsoft Windows HPC Server 2008 R2
Microsoft Windows Server 2008 Essential Business
Microsoft Windows Server 2008 Foundation Edition
Microsoft Windows Server 2008 R2
Microsoft Windows Server 2008 R2 Foundation Edition
Microsoft Windows Server 2008 W32
Microsoft Windows Server 2008 x64
Microsoft Windows Server 2012
Microsoft Windows Server 2012 R2
Microsoft Windows Small Business Server 2011 Standard and Essentials
Microsoft Windows Storage Server 2008 R2
Red Hat Enterprise Linux 6 Server (x86)
Red Hat Enterprise Linux 6 Server (x86-64)
SUSE Linux Enterprise Server 11 (AMD64/EM64T)
SUSE Linux Enterprise Server 11 (x86)
Ubuntu 12.04
VMware ESXi 5.0
VMware vSphere 5.1
VMware vSphere 5.5
So the idea of slapping Windows 7 on this machine for a quick and easy home server does not work. We tried and failed with Windows 7 x64; there are too many non compatible drivers.
http://www.silentpcreview.com/HP_ProLiant_MicroServer_Gen8