Well, actually, my XP install is brand new, and this is not the hard drive that is having trouble (doesn't emit any specific rattling) : it really looks like a bug.
I still don't think this is a performance issue, though : running on a A64 X2 4200+ rig equipped with 2 GB of RAM and a 256 MB Radeon X800XT, I shouldn't be victim of a performance dropdown.
Moreover, I had an eye on the task manager : when this bug happens, my CPUs are at 15-20% (due to window moving/drawing, I suppose), and memory usage graph doesn't increase a bit.
To me, it seems 2D video acceleration is suffering from MP!L (or WLM) handles. But I'm not an expert concerning this kind of issues, I just noticed this didn't happen with MSN 7.5 + MP!3.
Maybe something due to my dual screen environment and/or the way WLM handles window drawing?
Any hint, anyone?