**** Update ****
I decided to take a bet and to buy a new motherboard (P5E-VM with a ICH9R chipset) in the hope to solve the problem. It worked! I can now reach the expected fast read speed (250-260MB/s). One could think that it doesn't make a real difference in daily usage but its wrong! I can feel that everything is a little bit faster!
PS: Why not just go back with XP you may ask? After a few days with Win7 I could not go back with XP, it seemed suddenly very old and patched!