The other definitions are correct: Windows Server 2003 is very stable, and it is rather surprising that Microsoft could accomplish writing such an OS. It definitely pwns Mac OS X Server and is almost worthy of comparison with some Linux server systems.

An extremely stable and rock solid server operating system that is surprisingly made by microsoft. This software may allow windows to compete with linux and other better server OS's.

It is a successor to the Windows 2000 operating system that can be used both as a server and as a workstation, where a conversion may be done (although undesirable at the registry level - you essentially get a bloated XP then) by hand, preferably. The system runs indeed smoothly, but only if a person knows what he is doing. The system is more picky on various drivers' defects, so one has to adjust for example the service "Start" type from Auto (2) to On demand (3). The system also accepts nearly all Windows XP drivers (the Compatibility tab helps to the rest) so that no faux-pas seen on the Windows XP rise vs. older hardware can surprise you. You can download a Service Pack 1 for this system and integrate it into your installation to fix many bugs and enable the NX bit-based buffer overrun protection.
To return to the conversion to a desktop environment, I recommend tweaking in gpedit.msc a lot and copy nusrmgr.cpl from Windows XP to simplify the user management. You use the said Group Policy Edit...