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Although on a number of occasions I didn't and just decided for fun to fix it manually to see if it was still functional despite trace elements still in my registry. The other thing you may not have the ability to do now is image your OS during its freshly installed state since it has already been in use since then but had a clean image been done restoring it is so much easier than trying to eradicate and fix it post infection. However I find it unlikely if you're surfing Youtube and other legal sites banking or doing normal things only a badly led search engine result to some unknown site is the only way possible that it could happen but FireFox from my experience has never gotten infected that way although it is not impossible. The user is the usually the only one to infect themself by opening or running a file or downloaded attachment not knowing if it is legit and take the risk. The SP4 for the most part is a collection of security updates which even on SP3 I haven't once been hijacked using Firefox whereas it did happen with Internet Explorer like a boat with a new hole that pops up once another is plugged.
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Most XP software will only check for SP2 and SP3.
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Were I you, I'd reinstall SP3, make a full backup, apply all availble updates, make a second full backup, then add the POSReady trick and add the newly available updates, make yet another full backup, store the backups in a safe place and call it a day. Roffen, with all due respect, whatever benefits the SP4 might bring you are obviously outweighted by the amout of grief you've already got from it.
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