SP2 pretty much only updates the firewall and adds a pop up blocker. Be careful though, Ive heard if you dont have a legitimate copy of XP, it can lock you out of your own system.
Here is a brief list of updates items:

1. .dll files have been recompiled to take more advantage of 686 and 64bit processors

2. .dll files now have security buffer checks built into their code, and additional code to handle those checks on a processor level if the processor supports it on your computer.

3. key windows system files have the same two features as above

4. Massive amounts (last I checked, over 200+) of security fixes placed across the OS itself, including a good amount of them on the kernel level

5. Rewrite of certain parts of the kernel to optimize the buffer checking as well as secure itself against possible Stack crashes, and the implimentation of heavy memory permission checkings.



Sooo much more is in this SP than just the firewall. Just go to google and look for: sp2 whitepaper