The question is actually, do you experience any problems with your system? Updates are meant to fix something. I'm one of those guys - if it works, leave it
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However, SP3 is kinda worth the update, even though my PC's worked quite well with the SP2 version of XP. It just feels a little faster now. But I'd always recommend to start from scratch e.g. install Windows XP SP3, not just update it and watch how many unnecessary stuff you install next time as Windows get clogged [registry gets clogged actually] and slow rather quickly installing, reinstalling, uninstalling stuff. Registry is the most important part of Windows and if it gets clogged, you get a slow system. Keeping the registry as clean as possible is the best way to keep your Windows work nicely, the way like when you freshly installed it.
Cheers!
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