It's called sp3, and you can get it from here:
http://fileforum.betanews.com/detail/Microsoft_Windows_XP_Service_Pack_3/1197391546/1
Use nlite to slipstream updates, once sp3 is slipstreamed you will have only a few updates left which is .net ie7, wmp11 etc.
Or if you prepere you can simply download xp sp2 which can be downloaded via microsoft's own download site or a simple google search of "download windows xp sp2 or sp3"
Here is nlite's website. http://www.nliteos.com/
This is a great program which does it all automatically, it will make a new bootable iso at the end even. Simply select all updates, sp3, ie7, wmp11 etc, type in your product key all where applicable in the program, VERY EASY! I can't imagine life without it. You can even preinstall drivers on the new windows xp bootable cd!
It really is that simple and it has detail instructions.
It consists of the following, not necessarily in this exact order
1. download the updates executables from microsoft's own website or other download site. betanews, download.com or fileplanet etc.
2. Copy all the files from your current xp install cd to a folder on the desktop or other notable place, a simple drag and drop will do.
3. Download nlite, install it and run it.
4. It will ask what you want to do, you simply select the tasks unattend install, service packs, drivers, other hotfixes like ie7, wmp11 etc, then it will ask where the current system files are from the install cd, aka where you copied them on the computer via a browser button, the program will identify what version of xp you have with what service pack.
5. select the xp sp3 executable file when prompted in the program
6. Click next, and it will begin the process of integrating everything, when everything is done it will then make a bootable iso on the desktop, you simply burn this image with your favorite burning program. You can then try it out in the free pc imulation program virutalbox or virtual pc or something similiar just to varify that it works properly, boots up, what xp looks like etc, does it work properly
Excellent program, can't imaigne life without it!