It's been discussed before. I had a problem when I installed a few months ago on my XP Pro machine. Ended up taking a week on the phone with MS to resolve. The bottom line was that I hadn't downloaded all the OTHER updates before I installed SP2. MS said that not doing that caused MOST of the problem s that people had.
My problem was that I couldn't access Windows Update after the install. The way I fixed it was to uninstall and reinstall my sound card. Not sure if it was a newer version or not, though.