They are similar, but with a large number of differences. They also have a lot of similarities to the regular end-user, but there is a lot more that meets the eye between the two.
Windows Server 2003 is an upgrade of Windows 2000 Server. Windows XP was Microsoft’s move to provide an upgrade to various desktop/workstation OS versions like Windows 2000 Professional, Windows ME and Windows 98. In fact, you could not do straight upgrades from all these without a lot of adjustments.
•You can host other Windows components like DNS, Active Directory, and other Windows networking essentials on Windows 2003 server that you cannot do on Windows XP.
•You can not run Exchange (mail services) on Windows XP. You can run Outlook (mail client).
•There are many editions of Windows 2003 Server. There are only a few types of Windows XP (Home, Pro and Starter edition)
•Depending on the Windows 2003 Server edition, you can support many processors (CPUs) and much more RAM. To illustrate a few differences:
Windows XP 32-bit: 4 GB
Windows XP 64-bit: 128 GB
Windows 2003 Standard Edition 32-bit: 4 GB
Windows 2003 Standard Edition 64-bit: 16 GB
Windows 2003 Enterprise Edition 32-bit: 32 GB or 16 GB with 4 GT (not going to explain GT)
Windows 2003 Enterprise Edition 64-bit: 64 GB
•Network administration tools in Windows 2003 Server are non-existent in Windows XP.
•Number of users that can be defined to the Server vs. Windows XP (10 user limit)
You have other options for desktop and/or laptop workstations, but if you like and are used to Windows for day-to-day work, stay with XP.
I do not know why you asked, but if you are not going to be running software the demands Windows 2003 server, not hosting web sites, and not managing a domain, DNS, DHCP, stay with Windows XP. With all its flaws, Windows XP is a pretty good desktop/workstation OS as long a you keep it protected and do regular maintenance to it.
I am excited about learning Windows 7 and did not like Vista. Scratch that - I like Vista, but hope that my mother, sister, and a bunch of friends do not ever use it.....it is a bit more complex and requires more knowledge to take real advantage of it. Note this - all the newer OS' require better/faster computers.