[Mordekay]

The pick up weapon (if more powerfull) is a setting of cs itself. It does switch you to the picked up primary weapon (or secondary if you don't have a ny other weapon at all) if you have a pistol/knife/grenade in your hand. If this is turned off you drop your weapon (when you level up) automatically switch to knife (because you don't have any other weapon) and don't switch to the leveled weapon.