It appears you don't have to reinstall SP2 after all (sorry about that, although it would not have hurt if you did). I found this forum, which has a link that leads to Microsoft's Download Center with a Windows XP update (KB890831). You should note that this update isn't a security fix (or else it would say so in the title) and it's optional.

Unless if I'm mistaken, you should be able to uninstall this update through Add/Remove in your Control Panel. MSN Messenger should be the only thing affected if you remove this update, and unless you're using IME you won't notice any difference.