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).