IIRC you could actually run XP themes on 2k3 if you started the theme service. Under the hood they were almost the same exact same kernel (I think server / 5.2 had a slightly different IP stack and some server tweaks for task scheduling, shadow volumes and memory caching).
Likewise if you wanted the non XP (Luna I think it was called?) styles you could just stop the theme service and it was effectively win2k style.
> Under the hood they were almost the same exact same kernel
Literally the exact same kernel if you count XP x64 Edition. Even the exact same Windows Updates apply to both. :)
But yeah, Windows XP (32-bit; NT 5.1) and Server 2003 (NT 5.2) were pretty close. The latter was pretty much "XP Server Edition", albeit some significant kernel changes were made, especially to improve network performance.
It was the only time that the client and server versions of NT had diverged. Granted, the Windows Server team didn't feel like NT 6.0 was ready for the primetime when Vista launched, but instead of having two branches of the operating system again, Windows Server 2008 was launched identifying as "Service Pack 1" from the get-go, with Windows Vista's SP1 updating the OS to include all of the underlying improvements made for Server 2008. From that point forward, they were the same, including the same Windows Updates. It's also the point that Windows Vista got all of its technical flaws fixed. (Reputation flaws not withstanding, it wouldn't be until Windows Vista SP3 aka Windows 7, that the reputation would get reset...)
Likewise if you wanted the non XP (Luna I think it was called?) styles you could just stop the theme service and it was effectively win2k style.