I installed SP2 last night on my Windows XP Pro PC and boy did it crap out. During the final steps of the install it started giving me errors. Then it asked for a reboot. Upon doing so, the shell wouldn't load. I tried manually launching explorer with no luck. I did a safe start and a system restore to the previous day. Still, no explorer. I booted off the windows xp cd and did a repair. It failed too. I ended up reinstalling windows over my current installation, grabbing all my data files (.pst, my docs, etc...) and then formatting and starting over. I though perhaps my download of sp2 was bad, but after a clean install of windows I tried sp2 again and it went it smoothly.
I strongly recommend that you back up your critical files before running sp2 just in case you run into the same scenario as me.
Oh, and after all that I see that my website got hacked. Great.