Hey,
Thanks for the answer. My HD is a Samsung SpinPoint T - HD321KJ - 320 Go 7200 RPM, i defragmented it, used ccleaner, tried with ou without gamebooster, antivirus off etc. Video drivers are the latest.
The problem is the same on Arma 2, OA and DayZ.
I'm reading here and there that Arma2 is highly demanding CPU-wise. Maybe i should try overclocking ? I'm really out of options, mainly because i'm bad at computers.