If you haven't heard about it already I think you'd be interested in the [ReactOS](https://en.wikipedia.org/wiki/ReactOS#Development) project which has been working on building an OS that is binary compatible with Windows Server 2003 while being GPL licensed. It's far from daily drivable but it's a fascinating little project.

YES, Windows Server 2003R2 was the best version of Windows ever. I worked with it professionally for about 8 years, if you just used MS software on it (exchange sql server etc) that thing NEVER crashed, when we deployed a 2003R2 (as opposed to a 2008/2008R2) we referred to it as deploying the VMS (openVMS).

BTW: However, i never used the 2003 64-bit version.

Nothing stops you from using the classic theme on XP, and conversely, you can enable the eye-candy themes on 2003.

There was a 64bit XP around this timeframe as well and can use the classic theme.

Windows XP 64-bit is based on the Server 2003 code, funnily enough. My old ThinkPad didn't have a 64-bit CPU so I couldn't use it.

XP 64 was kinda good but crippled by poor driver support and thus stability issues.