Any version of Windows from Windows 95 and up includes software such that it can be a server - though perhaps with differing degrees of quality.
Windows XP Professional can make a very robust server given that it's based on the NT foundation.
Any Windows machine acting as a server that you don't use for any other purpose (such as running programs) becomes even more robust, thus an XP Pro machine dedicated as a server would be optimal.
A special 'Server' version of Windows such as Windows Server 2003 may become important once you have complex security requirements, large numbers of users, special software or the need to utilize multiple processors.
However, for almost all Shop-Pro users a 'Server' version of Windows is unnecessary and the added complexity can be a pain and can make general 'maintenance' more difficult.
If you need to mirror drives or use RAID and are concerned about Windows XP
support for that note that you can add a RAID card which will handle
RAID/ mirroring at a hardware level which is mostly independent of the
Windows version and likely with better performance.
Another benefit of running a normal version of Windows at the server is that in the case where the server fails, the users will be more able to quickly turn one of the other stations into the server on a temporary basis because the stations could be running the same type of Windows.
No matter the type of network configuration (windows, novell etc) the Shop-Pro software and data reside only on the server - not at each machine.