I've been using Win2k for many years. But these days some of the newer software released does not support Win2k (such as apple QuickTime). So I decided to upgrade to Win2k3 server (that's Windows 2003 server, in case you were wondering). Because I'm a student I can get Win2k3 for free from Microsoft, legally.
I don't really want to use it as a server tho - just as a replacement workstation OS for Win2k. Why not use XP or Vista (hah, Vista). Seriously, why not XP? Well, I built my own PC, and I didn't want to have to pay for a new OS. Since Win2k3 is free, the question is, can it be used as a desktop OS? There's already a website dedicated to this exact topic: http://win2k3.msfn.org/.
It appears that you can use Win2k3 as a desktop OS, with minimal adjustments to your settings. The above mentioned website is invaluable for that, though at times it suggests changes that are not necessary. For instance: The newest version of DirectX is already installed in Win2k3 SP2, and DirectX is already enabled by default, as is audio acceleration. But on the whole the website makes it very easy to make the conversion.
One question you might have is how hard it is to go back and forth between your new OS and your old OS? Perhaps you'll find that Win2k3 is not to your taste, and you don't want to trash your old OS install. Well, rest assured that like most other MS OSes, Win2k3 knows how to multiboot between OSes just fine. I was able to have Win2k and Win2k3 co-exists on the same computer just fine (though I did install them to separate hard drives).