|As far as an actual physical server, any of the HP Proliant series is great, they have great universal functionality and are easily repaired if they break down. |
As far as an actual operating system, it would all depend on how that program stores its data on the server and what the minimum requirements are. However, I would do some testing with various server OS's to find out what works and what doesn't. But if I had to guess, I would go with Windows Server 2008 Enterprise R2. This operating system has great functionality, has backwards compatibility and setting up a domain to get AD going is a breeze.
Also, if the server is going to act as a domain and file server, I would make sure the RAM is more than sufficient.
Jonas "Jackal" Lear
Serving the South Bay Since 1999