What OS to install?

I run the IT for a small business. All the workstations are Windows XP, we’ve got a Win 2k3 SBS server w/ Exchange, Fileserver, etc. on it. FTP & some backup systems are running on a CentOS server. I have an old SBS server that I’d like to be running some middleware that we need for a few things (that only runs on windows). However, I can’t join it to the domain due to the EULA restrictions. So I’m guessing I need to install something on it. Any thoughts? I appear to still be able to buy XP Pro, but is there a better option?

Happy to take any advice, but remember the restriction: I need Windows 2000 or newer to run the software I need this server to run.