Windows Server 2003
From BetaArchive Wiki
Windows Server 2003 (Windows NT 5.2.3790) , Released to Manufacturing on April 24 2003 is the server counterpart of Windows XP.
It's internal codenames were either the dotNET (.NET) Server (As evidenced in the Windows Buildtree, where it's source is located at D:\dnsrv) and earlier, Windows Whistler Server.
An interim release was made, under the name Windows Server 2003 R2 on December 6, 2005.