The installation of Orion NPM 8.1 will not work on a Windows 2008 server since it will not support the new IIS. In addition Windows 2008 support came with version Orion NPM 10.0.
Windows 2003 will work since it was supported at th etime when Orion NPM 8.1 came out.
If you are an existing customer sumit a support ticket in order to get your Orion NPM 8.1 upgraded to the latest version.
Otherwise you would need to revert to Windows 2003 server which Orion NPm 8.1 will support.