system
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How much free HDD space do you have (in each separate partition)?
Do you use some defrag tool?
Sometimes, updating drivers can help, but I guess you won’t find anything “revolutionary” for XP, and probably you don’t want to touch your XP for anything related to this (in spite some recommendations you already received, which I agree with).
Maybe it is avast, but we are trying to offer you suggestions for your particular system (workarounds), since I doubt avast will find what exactly is triggering this issue so to solve it (at least not in the near future).