For a home server you can use any desktop Windows (Windows 2000, Windows XP).
This is because most people require simple file sharing/streaming services which are basically trivial.
If you do require something more, the Windows user and vendor base is so immense that an application or service will be available to install. Maybe not for free, though.
The complexity of a server comes from managing data storage and protection, which is not necessarily down to just the OS.
But in the case of Windows Server Editions this is part of their job.
Hence you can get the newly released Windows Home Server, which is Server 2003 with 'simple to use' interface and configuration.
This is a newly growing market and set to grow even during the econmic downturn.
The other choice for a home server is Linux. This is free of monetary requirements but not 'time free'. Due to Linux being somewhat of a fringe system to home users it makes it harder to set up. However, there is always a way to accomplish anything in Linux and is more efficient and secure than Windows.
There are other exotic options like BSD, which are very good but not something home users might be willing to embrace for something trivial like a home server.
So the real choice for a home user is:
- Some version of Windows. You can get by with any version really but Windows Home Server is probably a sensible choice. Normal Server Editions are way too expensive and Desktop builds are not really optimal.
- Some version of Linux. I'd probably go with Debian as other folks have mentioned.