Microsoft ended Windows Server 2003’s Mainstream Support on July 13, 2010, and Extended Support on July 14, 2015. This means it would no longer provide security updates, technical support, or software updates for this server-based operating system.
Windows Server 2003 is probably my favourite Windows release. I never liked Windows XP, and Server 2003, with its updated codebase and various fixes compared to XP, provided a more solid alternative at the time. There was this whole cottage industry of people aiding each other in converting Windows Server 2003 into a more desktop-friendly operating system through reactivating services, installing additional components, applying registry changes, and so on. It was a bit of work post-install, but once done, you had a more stable, more solid, and safer “version” of Windows XP.
At least, that was the theory. I have no idea if this was actually true, or if a fully updated Windows XP installation was, in fact, functionally equivalent and that Server 2003 provided zero material benefit.