General use is noticebly more responsive, although I haven't ruled out the placebo effect yet. - Windows do seem to open quicker though.
Keep in mind that Windows in general (and XP I can 101% confirm, as I have it on two HDDs that I can boot from whenever I want) runs MUCH snappier (OS boots faster, IE opens faster, etc.) right after you just install it, compared to after a year or so of use when you clog the registry with software that you install/deinstall, etc. And no, it has nothing to do with viruses, spyware, etc. I have none of that - it's just that Windows simply 'degrades' that way after prolonged use, and you need to re-install it every now and then, just to keep performance at it's peak.
SP2 could be just refreshing the registry somehow, thus getting the same result that you get when reinstall from scratch.