As far as I am aware, there are no direct comparisons between XP Pro and Server 2003
Your question is kind of like comparing a standard pickup truck to a superheavy duty hauler
They're not considered at all in the same class because they're suited for very different purposes. Though they can do many of the same things, (just not to the same degree) and much of the underlying technology is the same
You wouldn't use XP Pro to host a SQL database or numerous networked printers...but you theoretically could. Nor would you use Server 2003 to get your POP mail with Outlook Express and play Alien vs. Predator...but, of course, you could
Server operating systems are designed to provide services for other computers to access...I.e. Printing, Active Directory, websites, databases and therefore the software is tailored to that purpose...as opposed to running applications like Word, Excel and E-mail
Then, of course, there's the price. There is several hundred dollars difference between the two. Server 2003 is anywhere from $700.00 and up depending on your source, vs. XP Pro which can be had for under $100.00 for an upgrade version
Hope this helps