If your goal is to use the Windows XP drivers in the session, then I'd try triCerat's Simplify Printing tool, which creates a virtual driver on the terminal server for that session's use. If your goal is just to eliminate having to install drivers on the terminal server, then there's a host of third-party tools that will allow you to avoid using a driver on the terminal server, among them RDP Engine by Thin Print and Print2RDP by BlackICE.
One question, though: you're sure that no printer drivers are available? I ask because you say that the clients are running Windows XP. I'd be surprised if a printer with support for Windows XP didn't have support for Windows Server 2003 -- you may need to download the driver from the Web site. It's possible, I suppose, and you may not want the hassle of supporting those individual printers.
This was first published in August 2006